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

layouts/_default/baseof.html

@@ -28,6 +28,7 @@
 {{ if $.Param "menuinheader" }}{{ partial "menu.html" . }}{{ end -}}
 </div>
 {{ if $.Param "mobilemenu" }}{{ partial "mobilemenu.html" . }}{{ end -}}
+{{ if $.Param "math" }}{{ partial "math.html" . }}{{ end -}}
 </header>
 
 {{ if not ($.Param "menuinheader") }}{{ partial "menu.html" . }}{{ end -}}