Add math typesetting support with KaTex (#80)

* add math typesetting support with KaTex
* Simplified the syntax for checking "math" property
* save katex files to local
* use local katex file to render math typesetting
* revert changes in scripts.html and styles.html
* Save math.js as a file, not inline script.
* move it into "assets/js" folder
* Load all necessary scripts and styles in math.html

exampleSite/config.yaml

@@ -40,3 +40,4 @@ params:
   realfavicongenerator: true
   sidebar: true
   submitted: true
+  math: true