Add code block syntax highlighting

[skydread1]

Closes #47

• Introduce the lib cljsjs/highlight
• Add a re-frame fx to perform the highlighting and a re-frame event to trigger the fx.
• Use Github syntax colors

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (90bec97) 90.54% compared to head (926aba1) 90.54%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   90.54%   90.54%           
=======================================
  Files           4        4           
  Lines          74       74           
=======================================
  Hits           67       67           
  Misses          7        7           

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