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
Safer addJumpHereE.
Merge remote-tracking branch 'benarmston/vim2-bdelete' into vim2
Fix some warnings.