Add configuration option to flatten components

[alerque]

See googlefonts/fontmake#714

Hasn't hit a release version yet, but when it does we should have a meta data configuration option so project can use it easier than fiddling with custom flag variables.

[alerque]

Now released in fontmake 2.3.0. Arch dependencies (which will affect Docker images) are struggling against an out of date python-fonttools package in [community], but support for this should still get ready.