Status of external subtitle files not correctly reported

`.srt` files are incorrectly reported as not indexed by Plex even though they may be. If there's a way to determine whether Plex is using these, then this should be fixed. Otherwise, perhaps a command-line option to ignore external subtitle files would be more convenient than a generic "exclude" option as proposed in #1?

Start by investigating how .srt files are classified and whether Plex exposes enough information to determine when external subtitles are indexed; also review the proposed generic exclude option in issue #1. Done means subtitle status is reported accurately, or a clearly scoped command-line option allows users to ignore external subtitle files.

Written by the indexing model from the issue text.