Add subsetting profiles

[wezm]

This is a continuation of #112. It reduces the scope to support @fschutt's use-case as well as Prince's use case. Future work that added support for subsetting layout tables (GSUB/GPOS #64) would allow the "full" profile to be brought back, which would address #27, but this is not planned at the moment.

In addition to profiles, the CmapTarget is exposed, which allows the cmap to be limited to Unicode cmaps. These are compatible with web browsers that reject fonts with only a Mac Roman cmap.

Closes #111

[wezm]

@fschutt does this work for you?

[fschutt]

I'll see. Thanks for your effort. I got it to work once, just in order to get font rendering to work for https://fschutt.github.io/printpdf/ - so I could decode fonts from PDF and create a new web font that just contains the characters being rendered.

[wezm]

These changes have been merged.