Add command to generate instances from feature flags

[alerque]

Many / most fonts have various features that can be enabled or disabled through OT feature flags. Unfortunately there are still quite a few applications out there that don't support this. Releasing every possible combination of features as separate fonts is silly and unrealistic to do by hand, but if we had a simple way to generate instances automatically or with a simple fontship command people could easily grab the ones they want.

See theleagueof/raleway#5 for an example.