[generic] Allow --default-search without colon

youtube_dl/__init__.py

@@ -255,8 +255,6 @@ def _real_main(argv=None):
         date = DateRange.day(opts.date)
     else:
         date = DateRange(opts.dateafter, opts.datebefore)
-    if opts.default_search not in ('auto', 'auto_warning', 'error', 'fixup_error', None) and ':' not in opts.default_search:
-        parser.error(u'--default-search invalid; did you forget a colon (:) at the end?')
 
     # Do not download videos when there are audio-only formats
     if opts.extractaudio and not opts.keepvideo and opts.format is None:

youtube_dl/extractor/generic.py

@@ -487,7 +487,8 @@ class GenericIE(InfoExtractor):
                      'Set --default-search "ytsearch" (or run  youtube-dl "ytsearch:%s" ) to search YouTube'
                     ) % (url, url), expected=True)
             else:
-                assert ':' in default_search
+                if ':' not in default_search:
+                    default_search += ':'
                 return self.url_result(default_search + url)
 
         url, smuggled_data = unsmuggle_url(url)