Implement initial concat support

It may not be pretty, but it friggin' works. Some initial testing with
The Daily Show, but I have no idea how it handles other sites.

youtube_dl/YoutubeDL.py

@@ -58,7 +58,7 @@ from .utils import (
 )
 from .extractor import get_info_extractor, gen_extractors
 from .downloader import get_suitable_downloader
-from .postprocessor import FFmpegMergerPP
+from .postprocessor import FFmpegMergerPP, FFmpegConcatPP
 from .version import __version__
 
 
@@ -640,6 +640,18 @@ class YoutubeDL(object):
                                                       extra_info=extra)
                 playlist_results.append(entry_result)
             ie_result['entries'] = playlist_results
+            
+            #Run concat PP
+            if self.params['concat']:
+                pp = FFmpegConcatPP(self)
+                
+                mergelist = []
+                for video in playlist_results:
+                    mergelist.append(video['saved_filename'])
+                ie_result['__files_to_append'] = mergelist
+                
+                pp.run(ie_result)
+            
             return ie_result
         elif result_type == 'compat_list':
             def _fixup(r):
@@ -811,13 +823,16 @@ class YoutubeDL(object):
             for format in formats_to_download:
                 new_info = dict(info_dict)
                 new_info.update(format)
-                self.process_info(new_info)
+                saved_filename = self.process_info(new_info)
+                #Maintain a list of saved files when processing playlists. Used by the concat PP.
+                info_dict['saved_filename'] = saved_filename
         # We update the info dict with the best quality format (backwards compatibility)
         info_dict.update(formats_to_download[-1])
+
         return info_dict
 
     def process_info(self, info_dict):
-        """Process a single resolved IE result."""
+        """Process a single resolved IE result. Returns the saved filename."""
 
         assert info_dict.get('_type', 'video') == 'video'
 
@@ -1019,6 +1034,7 @@ class YoutubeDL(object):
                     return
 
         self.record_download_archive(info_dict)
+        return filename
 
     def download(self, url_list):
         """Download a given list of URLs."""

youtube_dl/__init__.py

@@ -97,6 +97,7 @@ from .postprocessor import (
     FFmpegExtractAudioPP,
     FFmpegEmbedSubtitlePP,
     XAttrMetadataPP,
+    FFmpegConcatPP,
 )
 
 
@@ -495,6 +496,8 @@ def parseOpts(overrideArguments=None):
             help='"best", "aac", "vorbis", "mp3", "m4a", "opus", or "wav"; best by default')
     postproc.add_option('--audio-quality', metavar='QUALITY', dest='audioquality', default='5',
             help='ffmpeg/avconv audio quality specification, insert a value between 0 (better) and 9 (worse) for VBR or a specific bitrate like 128K (default 5)')
+    postproc.add_option('--join', action='store_true', dest='concat', default=False,
+            help='when downloading a playlist of multiple videos, try to join them together end-to-end (EXPERIMENTAL)')
     postproc.add_option('--recode-video', metavar='FORMAT', dest='recodevideo', default=None,
             help='Encode the video to another format if necessary (currently supported: mp4|flv|ogg|webm)')
     postproc.add_option('-k', '--keep-video', action='store_true', dest='keepvideo', default=False,
@@ -790,6 +793,7 @@ def _real_main(argv=None):
         'default_search': opts.default_search,
         'youtube_include_dash_manifest': opts.youtube_include_dash_manifest,
         'encoding': opts.encoding,
+        'concat': opts.concat,
     }
 
     with YoutubeDL(ydl_opts) as ydl:

youtube_dl/postprocessor/ffmpeg.py

@@ -488,13 +488,17 @@ class FFmpegMergerPP(FFmpegPostProcessor):
         return True, info
 
 class FFmpegConcatPP(FFmpegPostProcessor):
+    #Concat support requires that the IE return '_type' = 'playlist'
+    #Otherwise it silently fail
     def run(self, info):
-        filename = info['filepath']
+        filename = info['title'] + u'.mp4' #What could possibly go wrong?
         concatargs = ['-f', 'concat']
         args = ['-c', 'copy']
         self._downloader.to_screen(u'[ffmpeg] Appending files into "%s"' % filename)
+        #According to the ffmpeg docs this is literally how you're supposed to concat files.
+        #No method using solely the command line is listed. And I'm like "really?".
         with open(u'youtube-dl_ffmpeg_append_list.txt', 'wb') as f:
-            for file in info['__files_to_merge']:
+            for file in info['__files_to_append']:
                 f.write("file '" + file + "'\n")
         self.run_ffmpeg_multiple_files([u'youtube-dl_ffmpeg_append_list.txt'], filename, args, preopts=concatargs)
         os.unlink('youtube-dl_ffmpeg_append_list.txt')