Fixed plugin cards not using up width.

Introduced in 58c74d7b25642fcd4a861a01fd3f741ed3d79a8b.
Metadata plugin lists now stored as hashsets.

Closes #249.
Replaced Plugin hasher with std specialisation.
Split regex plugins into separate container.

Now they can get found correctly, they weren't getting matched in the
hashset. Timing puts having the hashset at about a half-second faster on
my computer than with just a list, so it is an improvement.