Add ':Qdo x' shortcut for ':Qargs | argdo x'

As contributed by "ib." here:
http://stackoverflow.com/questions/5686206/search-replace-using-quickfix-list-in-vim#comment8286582_5686810
Use arglocal/argglobal to keep global arglist intact
Merge pull request #1 from AndrewRadev/dont-clobber-args

Keep arglist intact
Make QuickfixFilenames script-local.
Make QuickfixFilenames function script-local

It's not used anywhere else.
If we're in the qf window, close it first

Otherwise some of the output will end up in the window.
Merge pull request #2 from AndrewRadev/avoid-using-quickfix-window

Avoid using quickfix window
Merge remote-tracking branch 'henrik/master'

* henrik/master:
  If we're in the qf window, close it first
  Make QuickfixFilenames function script-local
  Make QuickfixFilenames script-local.
  Use arglocal/argglobal to keep global arglist intact
  Add ':Qdo x' shortcut for ':Qargs | argdo x'

Conflicts:
	README.md
	plugin/qargs.vim