Bang ex command

Hi

I finally had some time to finish off work on the bang command.

1) Now uses PseudoTTY for 'range-less' bang command, so that output looks like a terminal
2) Uses a new quickfix 'window' which is an scrollable textview covering up to half of the bottom area of the editor window, and is dismissed by pressing a key, or losing focus.
3) Range-based bang commands now work with %, etc. instead of just '<,'>

I think my change is incompatible with Patrick's separate quickfix window change: I wanted to implement the vim way of opening a pane for the command-output. However, maybe both can be added as options.

Let me know if it needs any further tidying.

Best Regards
--Anthony