Support configuring preferred emoji presentation

[nicoburns]

Emoji often have both:

• A full-color pictoral representation (using emoji-specific font formats)
• A monotone textual representation (using regular glyph font formats)

Parley/Fontique should allow the user to specify a preference as a style and select a font with the appropriate representation during font fallback. Additionally, in some cases unicode characters U+FE0E and U+FE0F can be used to select a preference in the text stream. And Parley/Fontique should support a mode that selects the mode by using these characters.

See: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/font-variant-emoji

[behdad]

Chromium and some other systems use this ragel-generated parser for this:

https://github.com/google/emoji-segmenter

ragel's rust output can be used here.