Fix statusline highlighting after leaving a cmdwin

Because vim doesn't trigger any autocmds after leaving a cmdwin the
statusline in the window the user returned to from the cmdwin would be
highlighted as non-current even if it should have been current.

This issue is resolved by storing the last current window ID in a script
variable, and when leaving the cmdwin we show the last current window as
current instead of detecting it after the WinEnter autocmd has been
triggered (which doesn't happen after leaving a cmdwin).

Closes #184.