[subset] Add --glyph-file
[subset] Add --glyph-file

It only accepts PS glyph names, not gidNNN, U+XXXX, etc, as per
discussion here: https://github.com/behdad/fonttools/issues/113

Requested many times:
https://github.com/behdad/fonttools/pull/15
https://github.com/behdad/fonttools/pull/79

fonttools/fonttools,googlefonts/fonttools