Support Fira Code variants

[jesnor]

Really nice font that would be even better if it supported Fira Code variants (ssXX, cvXX etc.)

[somq]

Thanks @jesnor

All non-native characters including Ligatures and potentially variants come from directly from FiraCode font and are added by Ligaturizer.

To add variants we'd need to

1. Generate a FiraCode font with variants enabled via the feature flag (--features "ss02,ss08,ss10,cv03,cv07,cv14...")
2. Add the variants to the list of "ligatures" to be added to the font (list below)
3. Re-generate the font and publish it here

This issue might therefore belong to Ligaturizer for first steps, though I could take a look at it when I have some spare time (or maybe you would do a PR?).

Variants list for reference:

• https://github.com/tonsky/FiraCode/blob/master/extras/character_variants.png
• https://github.com/tonsky/FiraCode/blob/master/extras/ligature_variants.png