Add yml-based config files

It's painful to pass many arguments on the command-line. We should have a way to pass a yml file, similar to fontbakery:

https://github.com/googlefonts/fontbakery

We probably should have sections for sources (.glyphs, UFO, .fea, mti, ...), outputs to generate, and for each output (or globally), options like "remove overlap", "optimize", "fix outline directions", "autohint", etc.

In some distant future, this will become pluggable plugins / modules, and the optimizations in pyftsubset, as well as fixes that @dcrossland needs ala fontbakery, they all become part of fontmake configurable pipeline.

In some even more distant future, we'll configure pipelines using a data-flow graph...