add a --keep-everything flag to fonttools subset, like in hb-subset

[anthrotype]

@behdad added a --keep-everything option to hb-subset that forces subsetter to keep everything and can be overridden by subsequent options on the command line, e.g. if one wants to only drop specific unicodes or gids, etc.

A similar feature was requested several times e.g. #953 or #1153

see harfbuzz/harfbuzz#3998 for c++ implementation.