Add a KaTeX and MathJax based LaTeX pane

[philippjfr]

I think this should probably replace the LaTeX pane because it is much more capable. That said I still need to assess how expensive it would be to load KaTeX all the time, if it's too large I will likely have to let the user request it explicitly using pn.extension('katex') (not required on bokeh server or when saving).

[jbednar]

Looks great! Much better than my matplotlib-based hack. I'm happy for it to replace the LaTeX pane unless there are cases where the image-based approach is more general.

[philippjfr]

katex.min.js + katex.min.css are ~137 KB combined. Is it worth it to always bundle that or should I require the user to run pn.extension('katex') before using the component in the notebook? A helpful warning would tell the user they forgot to do so. I'm on the edge, I don't quite think using LaTeX is common enough to make everyone pay a 137 KB penalty. If I make it optional I could also allow toggling between MathJax and KaTeX and use whichever one is loaded.

[jbednar]

I'd say it's worth saving the 137KB at a cost of having to load the extension explicitly

[philippjfr]

Okay, I've now implemented MathJax as well and refactored in such a way that the LaTeX pane will use either the MathJax or the KaTeX model depending on which was loaded. If an explicit renderer is defined on the pane it will use that and if neither is loaded it will warn.

[codecov-io]

Codecov Report

Merging #311 into master will increase coverage by <.01%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   89.28%   89.29%   +<.01%     
==========================================
  Files          65       67       +2     
  Lines        6617     6622       +5     
==========================================
+ Hits         5908     5913       +5     
  Misses        709      709

Impacted Files	Coverage Δ
panel/models/katex.py	100% <100%> (ø)
panel/io.py	66.77% <100%> (ø)	⬆️
panel/pane/markup.py	92.85% <100%> (+0.1%)	⬆️
panel/tests/test_panes.py	93.84% <100%> (+0.4%)	⬆️
panel/models/mathjax.py	100% <100%> (ø)
panel/pane/equation.py	77.08% <84.61%> (-6.5%)	⬇️

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