Add kerning value based on exception types

[connordavenport]

Hey, I have a script for adding kerning based on exceptions so we dont need to know the group names off the top of our head. There was some interest on Discord to get this into fontParts. Here is what I came up with that I think should work well with the current implimentation.

connordavenport@1ea5504

Sorry I have no idea how to open an issue with a commit without doing a PR:)

CC @typesupply