initial LaTeX support (resolves #1)

This adds support for LaTeX rendering using KaTeX, a fast and popular
library. Since this is a custom extension to Markdown, all of this is
non-standard and doesn't work 100%, e.g. if you include math in some
<pre> code, it will get converted to HTML there (and shown to the user).

But otherwise it's pretty good, just use $ or $$ for inline or
block-based math, respectively.

lib/nbv.js

@@ -5,8 +5,8 @@ var nbv = (function() {
     var st = {}; // settings
 
     function render_ipynb(obj, target, settings) {
-        if (!window.marked || !window.Prism) {
-            console.error('expecting libraries marked.js and Prism.js to be present');
+        if (!window.marked || !window.Prism || !window.katex) {
+            console.error('expecting libraries marked.js, Prism.js and KaTeX to be present');
             return;
         }
         st = settings || {};
@@ -291,9 +291,18 @@ var nbv = (function() {
         return cn;
     }
 
+    function latexer(match, m1, offset, string) {
+        return katex.renderToString(m1, {
+            throwOnError: false, // we do not want to stop rendering because of bad LaTeX
+        });
+    }
+
     function handle_mdown(cell) {
         var el = d.createElement('div');
-        el.innerHTML = marked(cell.source.join(''));
+        var source = cell.source.join('');
+        var latexed = source.replace(/\$\$([\s\S]+?)\$\$/g, latexer); // block-based math
+        latexed = latexed.replace(/\$(.+?)\$/g, latexer); // inline math
+        el.innerHTML = marked(latexed);
 
         return el;
     }

viewer.html

@@ -9,6 +9,11 @@
     <script src='https://cdnjs.cloudflare.com/ajax/libs/prism/1.5.1/prism.min.js' data-manual></script>
     <script src='https://cdnjs.cloudflare.com/ajax/libs/prism/1.5.1/components/prism-python.min.js' data-manual></script>
 
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.css" integrity="sha384-zB1R0rpPzHqg7Kpt0Aljp8JPLqbXI3bhnPWROx27a9N0Ll6ZP/+DiW/UqRcLbRjq" crossorigin="anonymous">
+
+    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.js" integrity="sha384-y23I5Q6l+B6vatafAwxRu/0oK/79VlbSz7Q9aiSZUvyWYIYsd+qj+o24G5ZU2zJz" crossorigin="anonymous"></script>
+
+
     <style type='text/css'>
         body {
             font: 0.8em Arial, sans-serif;