Fix issues with ligatures in italic

This release uses a new way to patch the font files. It uses feature files (.fea) to build the necessary tables. It turns out that the fonttools package (the one that provides the ttx command) includes a way to import feature files.

This process is similar to how FIra Code works. I plan on supporting more ligatures and hopefully will be able to use this technique to handle some of the more advanced features as well (like unlimited arrows, context-sensitive spacing, etc.)