Handle & unit test all custom parameters

There are four lists of parameters to go through:
* those supported by Glyphs (e.g. listed in the Glyphs handbook): https://glyphsapp.com/downloads/handbook/Glyphs-Handbook-2.3.pdf
* those defined in UFO: http://unifiedfontobject.org/versions/ufo3/
* those that are understood by `ufo2ft`: https://github.com/googlei18n/ufo2ft
* those that are understood by `fontmake`: https://github.com/googlei18n/fontmake

For UFO and Glyphs parameters, we should have unit tests for each of them and decide where/under which key it is stored and how its value is converted when going from one format to the other.

Whenever the UFO standard doesn't support some Glyphs data, we should preferably translate it into a key/a format that `ufo2ft` or `fontmake` can process.

@moyogo

Start by comparing the Glyphs handbook, UFO 3 specification, ufo2ft, and fontmake parameter lists. Then inspect the repository's existing format-conversion code and tests to map each parameter, its storage key, and value conversion. Done means every relevant parameter has a defined handling decision and unit test, including fallbacks for Glyphs data unsupported by UFO.

Written by the indexing model from the issue text.