[YoutubeDL] Add generic video filtering (Fixes #4916)

This functionality is intended to eventually encompass the current format filtering.

youtube_dl/options.py

@@ -244,6 +244,25 @@ def parseOpts(overrideArguments=None):
         '--max-views',
         metavar='COUNT', dest='max_views', default=None, type=int,
         help='Do not download any videos with more than COUNT views')
+    selection.add_option(
+        '--match-filter',
+        metavar='FILTER', dest='match_filter', default=None,
+        help=(
+            '(Experimental) Generic video filter. '
+            'Specify any key (see help for -o for a list of available keys) to'
+            ' match if the key is present, '
+            '!key to check if the key is not present,'
+            'key > NUMBER (like "comment_count > 12", also works with '
+            '>=, <, <=, !=, =) to compare against a number, and '
+            '& to require multiple matches. '
+            'Values which are not known are excluded unless you'
+            ' put a question mark (?) after the operator.'
+            'For example, to only match videos that have been liked more than '
+            '100 times and disliked less than 50 times (or the dislike '
+            'functionality is not available at the given service), but who '
+            'also have a description, use  --match-filter '
+            '"like_count > 100 & dislike_count <? 50 & description" .'
+        ))
     selection.add_option(
         '--no-playlist',
         action='store_true', dest='noplaylist', default=False,