[subset] Add --text-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
[subset] Minor
[subset] Add --output-file

As requested many many times:

Fixes https://github.com/behdad/fonttools/pull/79
Fixes https://github.com/behdad/fonttools/pull/15
Fixes https://github.com/behdad/fonttools/issues/44