HasMask capability

[aherrmann]

Should we add a HasMask capability to complement HasThrow, and HasCatch, similar to MonadMask?

If so, what would the meaning of the tag be? Should it only select a layer in a transformer stack (if applicable), or should it decide which exceptions to mask? The latter seems to contradict the purpose of bracket.

See #25 (comment).

cc @aspiwack