Add :Acks

Run the specified substitution command on all the files in the quickfix
list (mnemonic: "Ack substitute").

Specifically, the sequence:

  :Ack foo
  :Acks /foo/bar/

is equivalent to:

  :Ack foo
  :Qargs
  :argdo %s/foo/bar/ge | update

(Note: there's nothing specific to Ack in this function; it's just named
this way for mnemonics, as it will most often be preceded by an :Ack
invocation.)