[core] add option to trim file name length with integer

https://github.com/blackjack4494/youtube-dlc/issues/85

youtube_dlc/options.py

@@ -775,6 +775,9 @@ def parseOpts(overrideArguments=None):
         '--rm-cache-dir',
         action='store_true', dest='rm_cachedir',
         help='Delete all filesystem cache files')
+    filesystem.add_option(
+        '--trim-file-name', dest='trim_file_name', default=0, type=int,
+        help='Limit the filename length (extension excluded)')
 
     thumbnail = optparse.OptionGroup(parser, 'Thumbnail images')
     thumbnail.add_option(