Robust wildcard support, including exclusion

My attempt at a fix of #70. Brings exclusions starting with `-` in line with inclusions as far as the wilcard `*` character when searching for posts.

Also fixes a pre-existing bug whereby `_` was being parsed as it usually is as a single wildcard character whenever a `*` was present. For example, searching for `under_sc*res` was returning results tagged not only `under_scores` but also `under9scores`, `underXscores`, etc. This has been resolved.

No known issues.