[fcformat] Add element filtering and deletion

[fcformat] Add element filtering and deletion

The filtering, '%{+elt1,elt2,elt3{subexpr}}' will evaluate subexpr
with a pattern only having the listed elements from the surrounding
pattern.

The deletion, '%{-elt1,elt2,elt3{subexpr}}' will evaluate subexpr
with a the surrounding pattern sans the listed elements.