Sub- and super-scripts

[gustaphe]

This is a re-PR of #167 , rebased on current master and with two added kwargs which I feel address the slight regressions in that PR by way of customizability:

julia> @latexify hello_world
L"$hello_{world}$"

julia> @latexify hello_world snakecase=true
L"$hello\_world$"

julia> @latexify aᵦᵧ¹²₃
L"$a_{\beta \gamma}^{1 2}_3$"

julia> @latexify aᵦᵧ¹²₃ safescripts=true
L"$a{_{\beta \gamma}}{^{1 2}}{_3}$"

(A majority of this is @akabla's work, I just couldn't figure out how to merge this otherwise)

[codecov]

Codecov Report

Merging #215 (ffc933c) into master (6bf3875) will decrease coverage by 0.21%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   62.90%   62.68%   -0.22%     
==========================================
  Files          23       23              
  Lines         806      812       +6     
==========================================
+ Hits          507      509       +2     
- Misses        299      303       +4     

Impacted Files	Coverage Δ
src/unicode2latex.jl	89.74% <88.46%> (-10.26%)	⬇️
src/latexoperation.jl	84.78% <100.00%> (ø)
src/latexraw.jl	90.62% <100.00%> (ø)

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