Unlet b:Colorizer_force after ColorWinEnter

After`:ColorHighlight`, DoColor (via ColorWinEnter) gets called always,
although b:changedtick is unchanged.

It seems like b:Colorizer_force needs to be unset, after it initially
caused the coloring.

There is an issue left still: after switching away and back to the
window `:ColorHighlight` had been called, it gets triggered once again.
Fix detection of existing buffer autocommands

Without this, the buffer gets up setup again and again.
Drop b:Colorizer_force: call ColorWinEnter with force instead

The meaning of b:Colorizer_force appears to have been setting up a
window initially.

This changes it to call ColorWinEnter(1) once instead.

This also improves the check for force in ColorWinEnter slightly.
Merge pull request #29 from blueyed/only-force-once

Only force once