Completion window stays after backspace.

Changed the way the backspace command is handled to bring back
the selections that were filtered out by the deleted keystroke.

Also set the window to be dismissed when the user starts deleting
into the prefix.
Merge branch 'master' into keep-completion-window-after-backspace
Created the backspace key binding.
Removed an empty line. Because that's improtant.
Removed useless comment.
Merge pull request #57 from nomothetis/keep-completion-window-after-backspace

Keep completion window after backspace.