Option to ignore short unprefixed inline issue numbers

[dgw]

It really seems like most projects that seriously use a plugin like this are going to have at least double-digit issues as the active set. Parsing the message <thing> is my #1 favorite into an issue is never useful in Sopel's own channel.

Related to #127 but not the same (that's for a heuristic; this is for a configuration setting/s).

It's possible that this would be better as a per-channel value like the "linked repo". A global setting is simpler, though.