Make it easier to build variant families

[simoncozens]

We currently have support in the builder for building small cap variant families; this generalises that idea, with a new configuration option to run a list of operations after the initial font compile. It also adds an option to add a suffix to the output filenames.

Now instead of having to cook up a custom recipe from scratch to handle, say "Cormorant Garamond" (Cormorant + baked-in ss02 feature), you can instead create a separate configuration file like so:

sources:
  - Cormorant.glyphs
  - Cormorant-Italic.glyphs
filenameSuffix: Garamond
postCompile:
  - operation: remapLayout
    args: '''ss02 -> ccmp'''
  - operation: rename
    args: --just-family
    name: Cormorant Garamond