Add id attributes to heading elements

This changes the generated HTML to include `id` attributes on any heading (e.g. `h3`) elements.

Currently, `bit-docs-html-toc` will add `id` attributes when it parses the content and creates the TOC links. This means that JS has to be downloaded, parsed, and run before `bit-docs-html-toc` runs its code, and _then_ there has to be more JS to scroll down to the right fragment.

This takes JS completely out of the equation by including the `id` attributes in the source HTML, so JS doesn’t need to load and `bit-docs-html-toc` isn’t required to make fragment URLs work either.

Part of https://bitovi.atlassian.net/browse/LD-203

html_test.js

@@ -10,6 +10,7 @@ var readFile = Q.denodeify(fs.readFile);
 var bitDocsHTML = require("./bit-docs");
 
 require("./build/build_test");
+require("./stmd_test");
 
 describe("bit-docs-generate-html", function(){
 	beforeEach(function(){

stmd.js

@@ -1476,8 +1476,12 @@ var renderBlock = function(block, in_tight_list) {
                     this.innersep);
     case 'ATXHeader':
     case 'SetextHeader':
+      var rendered = this.renderInlines(block.inline_content);
       tag = 'h' + block.level;
-      return inTags(tag, [], this.renderInlines(block.inline_content));
+      attr = [
+        ['id', makeHeadingId(rendered)]
+      ];
+      return inTags(tag, attr, rendered);
     case 'IndentedCode':
       return inTags('pre', [],
               inTags('code', [], this.escape(block.string_content)));
@@ -1544,4 +1548,11 @@ function HtmlRenderer(){
 exports.DocParser = DocParser;
 exports.HtmlRenderer = HtmlRenderer;
 
-})(typeof exports === 'undefined' ? this.stmd = {} : exports);
+})(typeof exports === 'undefined' ? this.stmd = {} : exports);
+
+function makeHeadingId(text) {
+	return (text || "")
+		.replace(/\s/g, "-")      // replace spaces with dashes
+		.replace(/[^\w\-]/g, "")  // remove punctuation
+		.toLowerCase();
+}

stmd_test.js

@@ -0,0 +1,14 @@
+var assert = require('assert');
+
+var stmd = require("./stmd");
+
+describe("bit-docs-generate-html/stmd", function () {
+	it("adds id attributes to heading elements", function () {
+		var parser = new stmd.DocParser();
+		var renderer = new stmd.HtmlRenderer();
+		var markdown = "# Hello world"
+		var rendered = renderer.render(parser.parse(markdown));
+		var expected = '<h1 id="hello-world">Hello world</h1>';
+		assert.equal(rendered.trim(), expected, "id attributes are present");
+	});
+});