AviUtl/vpy_input: Try to get color property only when it's present.
Add install and uninstall recipes to VapourSynth plugin makefile
Clean up the Git-based version info generation a bit.

'git describe' can be fragile about what it reports, and using
rev-list's --count option removes the need to use wc, which also
means the use of sed to clean up wc's output on OS X is no longer
necessary either.