Better support for math fonts

[Kolaru]

Fix #49

Also improve the tree printing of TeXExpr.

[codecov-commenter]

Codecov Report

Merging #72 (d4b06fe) into master (4ead598) will increase coverage by 0.05%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   78.74%   78.80%   +0.05%     
==========================================
  Files           7        7              
  Lines         447      467      +20     
==========================================
+ Hits          352      368      +16     
- Misses         95       99       +4     

Impacted Files	Coverage Δ
src/parser/parser.jl	73.68% <ø> (ø)
src/parser/texexpr.jl	80.00% <78.26%> (ø)
src/engine/layout.jl	89.60% <100.00%> (ø)

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