Delete some unused code from Yi.Keymap
Implement some missing pieces in Ex command editing.
Shuffle imports in Yi.Keymap to fix compilation with GHC 7.4
Use Yi.Keymap.Action instead of Either (EditorM ()) (YiM ()) in Ex commands.
Fix blockvisual indent and delete.
Fix for closing the only window in the only tab

Previously, closing the only window in the only tab raised an error due to
calling `fromJust` on a `Nothing`. Now we do what the documentation said we
did, and do nothing when there is a single window and a single tab.
Add :bdelete command