Add UnicodeMath.jl -- mimic LaTeX package unicode-math for more control over font styles

[manuelbb-upb]

This is a follow-up to #141

I have factored out the UnicodeMath stuff into a new package.
It is awaiting registration.

Right now, I have only added the \symxx commands, not the additional commands that unicode-math also defines.

Still a WIP.

[manuelbb-upb]

I think the current state is nice, but should be considered breaking.

I have removed any Unicode glyph substitutions during parsing.
For styling glyphs, there are now the \textXX, \mathXX and \symXX commands.
The text commands are not changed -- as before they are font and state switches.
The \symXX commands should replace the previous \mathXX commands.
Like with the unicode-math LaTeX package, rather than switching fonts, stylized Unicode glyphs are chosen from the :math font.
By default, the \mathXX command alias the \symXX commands.
But this behavior is configurable with a new field mathfont_command_mapping in the FontFamily.

Instead of referring to global Ref objects, the behavior of the \symXX commands is now determined by the new fields unicode_math_substitutions and unicode_math_aliases in a FontFamily.

Right now, UnicodeMath is a submodule and we could consider keeping it that way.