Add control over math rendering in HTML and Markdown panes

[philippjfr]

Fixes #374

[codecov]

Codecov Report

Merging #2847 (ba26da5) into master (1a334dc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2847   +/-   ##
=======================================
  Coverage   83.05%   83.06%           
=======================================
  Files         188      188           
  Lines       24450    24452    +2     
=======================================
+ Hits        20307    20311    +4     
+ Misses       4143     4141    -2     

Impacted Files	Coverage Δ
panel/pane/markup.py	97.22% <100.00%> (+0.03%)	⬆️
panel/tests/command/test_serve.py	85.00% <0.00%> (+2.50%)	⬆️

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