Missing ack compatibility for ignoring filetypes with `--no<filetype>`

[dkaszews]

With filetypes cc: .c, .h, .xs and hh: .h, I can combine them as ack --cc --nohh to search only C implementation files but not headers. Trying to do the same with ag results in error "unknown option -- nohh". Alternative syntax such as --types +cc,-hh or similar would also be satisfactory.