Add KaTex math rendering

Add KaTex javascript, CSS, and fonts to vendor folder.

Create a renderMath() function that parses sanitized markdown for math as denoted by '$$'

Modify ctrl.$render to first sanitize the markdown and then renderMath. Newly rendered
math is then trusted as HTML becuase otherwise ngSanatize removes crucial style attributes
that leave the math unreadable.

h/static/scripts/directives/markdown.coffee

@@ -7,7 +7,7 @@
 # the markdown editor.
 ###
 
-markdown = ['$filter', '$timeout', ($filter, $timeout) ->
+markdown = ['$filter', '$sanitize', '$sce', '$timeout', ($filter, $sanitize, $sce, $timeout) ->
   link: (scope, elem, attr, ctrl) ->
     return unless ctrl?
 
@@ -251,10 +251,39 @@ markdown = ['$filter', '$timeout', ($filter, $timeout) ->
           input.style.height = output.style.height
           $timeout -> inputEl.focus()
 
+    scope.renderMath = (textToCheck) ->
+      # Parses text for math as denoted by '$$'
+      i = 0
+      startMath = null
+      endMath = null
+      for char in textToCheck
+        if char == "$" and textToCheck[i + 1] == "$"
+          if startMath == null
+            startMath = i + 2
+          else
+            endMath = i
+        i++
+        if startMath != null and endMath != null
+          try
+            math = katex.renderToString(textToCheck.substring(startMath, endMath))
+          catch error
+            # Show error on the frontend so users have some insight of which Tex Commands
+            # caused the error. KaTex does not yet support all Tex Commands.
+            math = error
+          textToCheck = textToCheck.substring(0, (startMath - 2)) + math + textToCheck.substring((endMath + 2))
+          startMath = null
+          endMath = null
+          scope.renderMath(textToCheck)
+      return textToCheck
+
     # Re-render the markdown when the view needs updating.
     ctrl.$render = ->
-      inputEl.val (ctrl.$viewValue or '')
-      scope.rendered = ($filter 'converter') (ctrl.$viewValue or '')
+      if !scope.readonly and !scope.preview
+        inputEl.val (ctrl.$viewValue or '')
+      value = ctrl.$viewValue or ''
+      markdown = $sanitize $filter('converter') value
+      rendered = scope.renderMath markdown
+      scope.rendered = $sce.trustAsHtml rendered
 
     # React to the changes to the input
     inputEl.bind 'blur change keyup', ->

h/static/scripts/vendor/katex/README.md

+# [<img src="https://khan.github.io/KaTeX/katex-logo.svg" width="130" alt="KaTeX">](http://khan.github.io/KaTeX/) [![Build Status](https://travis-ci.org/Khan/KaTeX.svg?branch=master)](https://travis-ci.org/Khan/KaTeX)
+
+KaTeX is a fast, easy-to-use JavaScript library for TeX math rendering on the web.
+
+ * **Fast:** KaTeX renders its math synchronously and doesn't need to reflow the page. See how it compares to a competitor in [this speed test](http://jsperf.com/katex-vs-mathjax/).
+ * **Print quality:** KaTeX’s layout is based on Donald Knuth’s TeX, the gold standard for math typesetting.
+ * **Self contained:** KaTeX has no dependencies and can easily be bundled with your website resources.
+ * **Server side rendering:** KaTeX produces the same output regardless of browser or environment, so you can pre-render expressions using Node.js and send them as plain HTML.
+
+KaTeX supports all major browsers, including Chrome, Safari, Firefox, Opera, and IE 8 - IE 11.
+
+## Usage
+
+Download the built files from [the releases page](https://github.com/khan/katex/releases). Include the `katex.min.js` and `katex.min.css` files on your page:
+
+```html
+<link rel="stylesheet" type="text/css" href="/path/to/katex.min.css">
+<script src="/path/to/katex.min.js" type="text/javascript"></script>
+```
+
+Call `katex.render` with a TeX expression and a DOM element to render into:
+
+```js
+katex.render("c = \\pm\\sqrt{a^2 + b^2}", element);
+```
+
+To generate HTML on the server, you can use `katex.renderToString`:
+
+```js
+var html = katex.renderToString("c = \\pm\\sqrt{a^2 + b^2}");
+// '<span class="katex">...</span>'
+```
+
+Make sure to include the CSS and font files, but there is no need to include the JavaScript.
+
+## Contributing
+
+See [CONTRIBUTING.md](CONTRIBUTING.md)
+
+## License
+
+KaTeX is licenced under the [MIT License](http://opensource.org/licenses/MIT).