options: Add enable_subtitles config option

Enable generic subtitle support by default for unknown clients.  Set the
new enable_subtitles option to false to revert to the old behavior.

options.h

@@ -60,6 +60,7 @@ enum upnpconfigoptions {
 	MERGE_MEDIA_DIRS,		/* don't add an extra directory level when there are multiple media dirs */
 	WIDE_LINKS,			/* allow following symlinks outside the defined media_dirs */
 	TIVO_DISCOVERY,			/* TiVo discovery protocol: bonjour or beacon. Defaults to bonjour if supported */
+	ENABLE_SUBTITLES,		/* Enable generic subtitle support for all clients by default */
 };
 
 /* readoptionsfile()