Add CLI to fontmake.instantiator

[simoncozens]

This adds a CLI interface to the instantiator; useful for testing/debugging and also actually instantiating UFOs!

Can be used either with an instance name:

% python3 -m fontmake.instantiator --output-dir instance_ufo GlyphsUnitTestSans.designspace Light
Generated instance 'Light'.
Saved to 'instance_ufo/GlyphsUnitTestSans-Light.ufo'.

or with coordinates:

% python3 -m fontmake.instantiator --output-dir instance_ufo GlyphsUnitTestSans.designspace wght=523
Generated instance 'wght=523'.
Saved to 'instance_ufo/GlyphsUnitTestSans-wght523.ufo'.

[anthrotype]

doesn't the fontmake CLI already allow you to make instance UFOs and select specific named instances? Maybe we could simply add an option to instantiate at some arbitrary location..

[anthrotype]

i worry that we may then have to support multiple ways to accomplish the same thing, that's all

[simoncozens]

That does make sense. I was seeing instantiator as a self-contained bit of code, but of course within fontmake it can do almost all of this.

[anthrotype]

I actually wanted to move that module out of fontmake (which I see just as the command line front-end) to ufo2ft (which is a library doing most of the building logic), but never got around doing that.

[simoncozens]

Could we make it a separate project altogether? That way if we ever need to replace the instantiator with a different implementation it's all nicely compartmentalised...

[anthrotype]

yeah probably better that way