Woff2 support

[shuding]

Currently opentype.js doesn't support woff2, which is usually 30% the size of woff. Fontkit might be a good alternative for this (but the lib is larger).

Ref: opentypejs/opentype.js#183

[shuding]

Another advanced feature that Fontkit supports is color emoji glyphs, downside is Fontkit is Node.js only (using Buffer and fs). Options:

• https://github.com/diegomura/react-pdf/tree/master/packages/fontkit
• https://github.com/Hopding/fontkit

[shuding]

Decided to put a lower priority on this. TTF & OTF are faster to load and parse, so on the server side we will always recommend using them unless there's a size limitation. WOFF is a good balance of size and parsing speed (smaller but slightly longer to load).