Qargs to do commands on quickfix lists.

The main use of this is to have a faster and more efficient solution
than `Greplace` which doesn't perform well when the amount of files is
big.

Where `Greplace` freezes, `Qargs | Argdo` succeeds.

Example:

    :Ack! 'some-world'
    :Qargs | Argdo %s/some/hello/g
Merge remote-tracking branch 'mario/qargs' into magus

* mario/qargs:
  Qargs to do commands on quickfix lists.