Add '--expand-features-to-instances' arg to fontmake UFO generation

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UFOs can use relative include statements to import feature files, which is very helpful in the case of a family that has many UFO sources, but just one feature file required. Or, it’s helpful if there are many features, and several complex ones, so they are split into multiple files (example: a Recursive UFO source and its features directory).

Currently, the Builder just brings the include statement directly into the instance UFOs, but then the relative paths are broken, so the fonts don’t build.

fontmake: Error: Compiling UFO failed: <features>:1:8: The following feature file should be included but cannot be found: ./features/features.fea

However, it is very easy to support by adding the arg --expand-features-to-instances to the UFO instantiation step, as I have done in this PR.

I’ve tested this on a UFO-based family (with includes) and a Glyphs-based family (with normal Glyphs-based features), and it seems to work well in both cases. The features are built into the fonts, as expected.

I’m not aware of any downsides or hidden issues with adding this, but they might exist. If any are known, I would be very interested in learning about such cases.

Please let me know if you have any questions or suggestions here! Thanks so much.