Fix vim keymap behavior regarding :q.

:q, :q!, and :qa! were all misbehaving. Fix :q to error if the file is
modified and unsaved, and have :q! not close all windows. :qa! now
simply closes all windows without checking for unsaved changes in
buffers.
Merge pull request #560 from jhance/vim-ex-quit

Fix vim keymap behavior regarding :q.