Prevent some episode numbers from being treated as quality

Flexget apparently doesn't correctly handle episode numbers that look like a quality. For instance when presented "[HorribleSubs] Naruto Shippuuden - 368 [720p].mkv", it strips BOTH 368 and 720p from the string, which leads to "cannot find a(n) `sequence` style identifier" later.

This patch tries to work around that by preventing stripping of all matched resolution-like strings, and instead stripping only the resolution that is returned as a result from _find_best(). This can be deemed a non-invasive change, b/c it's unlikely that there will be several resolutions mentioned in entry at once (e.g. 720p 480p). This is not so e.g. for codecs, where there can be e.g. h264 hi10p, but this patch doesn't alter the parsing of that.