add --remove-overlap option

We already have a `--keep-overlap` option to skip removing the overlaps while building OTF or TTF.

The overlap removal only happens upon generating binary font formats, not when generating UFOs from glyphs or designspace input.

Sometimes it could be useful to generate instance UFOs with the overlap removal applied. We could do that by adding a `--remove-overlap` option that does that when combined with UFO output `-o ufo`.