Add Parser arg taking a callable to filter element-wise

[bskinn]

E.g., "drop anything containing a colon", for mwfn LI block.

Might not be too generally useful, since it's probably(?) impractical to target it to a specific capture token...unless adding a 'key' field to the token would then allow matching of the key to the passed callable...mapping of keys to callable, perhaps.

Would get pretty hairy...multiple keys on a token, cross-application of different callables...could be very powerful, though.