Add text/emoji variation selectors

[13rac1]

The fonts default to emoji representation right now, which is technically incorrect. The text and emoji variation selectors should be implemented using font ligatures to correctly select text or emoji versions as requested.

The emoji style variants are all produced with the addition of U+FE0E VARIATION SELECTOR-15 (VS15) for a text presentation or U+FE0F VARIATION SELECTOR-16 (VS16) for an emoji presentation.

Details:

• http://www.unicode.org/Public/UCD/latest/ucd/StandardizedVariants.txt
• http://www.unicode.org/Public/emoji/2.0/emoji-data.txt

[13rac1]

Hmm... When this is implemented I'll need to get some regular "text" glyphs to use as the text presentation. The DejaVu ones could work.

[13rac1]

An example of why the variation selectors are needed: https://www.reddit.com/r/space/comments/4npqj5/the_size_of_hubbles_deep_field_image_in_relation/d46d0zs

Specific details: https://en.wikipedia.org/wiki/Arrows_(Unicode_block)
http://unicode.org/Public/UCD/latest/ucd/StandardizedVariants.txt

[13rac1]

This makes it more clear which is which: http://unicode.org/reports/tr51/proposed.html#Presentation_Style

[13rac1]

Project EOL, issue moved to: 13rac1/twemoji-color-font#39