Add footnote parsing/rendering support

Fixes GitbookIO/gitbook#395, fixes GitbookIO/gitbook#134, fixes
GitbookIO/gitbook#237

Based off markedjs/marked#431

lib/kramed.js

@@ -27,6 +27,7 @@ var block = {
   list: /^( *)(bull) [\s\S]+?(?:hr|def|\n{2,}(?! )(?!\1bull )\n*|\s*$)/,
   html: /^ *(?:comment *(?:\n|\s*$)|closed *(?:\n{2,}|\s*$)|closing *(?:\n{2,}|\s*$))/,
   def: /^ *\[([^\]]+)\]: *<?([^\s>]+)>?(?: +["(]([^\n]+)[")])? *(?:\n+|$)/,
+  footnote: /^\[\^([^\]]+)\]: ([^\n]+)/,
   table: noop,
   paragraph: /^((?:[^\n]+\n?(?!hr|heading|lheading|blockquote|tag|def|math))+)\n*/,
   text: /^[^\n]+/,
@@ -43,6 +44,7 @@ block.list = replace(block.list)
   (/bull/g, block.bullet)
   ('hr', '\\n+(?=\\1?(?:[-*_] *){3,}(?:\\n+|$))')
   ('def', '\\n+(?=' + block.def.source + ')')
+  ('footnote', block.footnote)
   ();
 
 block.blockquote = replace(block.blockquote)
@@ -205,6 +207,17 @@ Lexer.prototype.token = function(src, top, bq) {
       continue;
     }
 
+    // footnote
+    if (cap = this.rules.footnote.exec(src)) {
+      src = src.substring(cap[0].length);
+      this.tokens.push({
+        type: 'footnote',
+        refname: cap[1],
+        text: cap[2]
+      });
+      continue;
+    }
+
     // math
     if (cap = this.rules.math.exec(src)) {
       src = src.substring(cap[0].length);
@@ -475,6 +488,7 @@ var inline = {
   link: /^!?\[(inside)\]\(href\)/,
   reflink: /^!?\[(inside)\]\s*\[([^\]]*)\]/,
   nolink: /^!?\[((?:\[[^\]]*\]|[^\[\]])*)\]/,
+  reffn: /^!?\[\^(inside)\]/,
   strong: /^__([\s\S]+?)__(?!_)|^\*\*([\s\S]+?)\*\*(?!\*)/,
   em: /^\b_((?:__|[\s\S])+?)_\b|^\*((?:\*\*|[\s\S])+?)\*(?!\*)/,
   code: /^(`+)\s*([\s\S]*?[^`])\s*\1(?!`)/,
@@ -496,6 +510,10 @@ inline.reflink = replace(inline.reflink)
   ('inside', inline._inside)
   ();
 
+inline.reffn = replace(inline.reffn)
+  ('inside', inline._inside)
+  ();
+
 /**
  * Normal Inline Grammar
  */
@@ -651,6 +669,13 @@ InlineLexer.prototype.output = function(src) {
       continue;
     }
 
+    // reffn
+    if ((cap = this.rules.reffn.exec(src))) {
+        src = src.substring(cap[0].length);
+        out += this.renderer.reffn(cap[1]);
+        continue;
+    }
+
     // reflink, nolink
     if ((cap = this.rules.reflink.exec(src))
         || (cap = this.rules.nolink.exec(src))) {
@@ -899,6 +924,19 @@ Renderer.prototype.del = function(text) {
   return '<del>' + text + '</del>';
 };
 
+Renderer.prototype.reffn = function(refname) {
+  return '<sup><a href="#fn_' + refname + '" id="reffn_' + refname + '">' + refname + '</a></sup>'
+}
+
+Renderer.prototype.footnote = function(refname, text) {
+  return '<blockquote id="fn_' + refname + '">\n'
+    + '<sup>' + refname + '</sup>. '
+    + text
+    + '<a href="#reffn_' + refname + '" title="Jump back to footnote [' + refname + '] in the text."> &#8617;</a>\n'
+    + '</blockquote>\n';
+}
+
+
 Renderer.prototype.link = function(href, title, text) {
   if (this.options.sanitize) {
     try {
@@ -1018,6 +1056,11 @@ Parser.prototype.tok = function() {
         this.token.depth,
         this.token.text);
     }
+    case 'footnote': {
+      return this.renderer.footnote(
+        this.token.refname,
+        this.inline.output(this.token.text));
+    }
     case 'code': {
       return this.renderer.code(this.token.text,
         this.token.lang,