Toggle numbers on/off on WinEnter/WinLeave

When working with multiple split windows, it makes more sense for only
the active window to be in relative number mode and for all the other
windows to be in absolute number mode.
Documentation update for active window only
Merge pull request #28 from mrpdaemon/toggle-on-window-enter-leave

Toggle numbers on/off on WinEnter/WinLeave