feat(converter.makeMarkdown): add an HTML to MD converter

Showdown now supports a simple HTML to Markdown converter.

**Usage:**

```
var conv = new showdown.Converter();
var md = conv.makeMarkdown('<a href="/url">a link</a>');
```

Closes #388, #233

Gruntfile.js

@@ -25,6 +25,7 @@ module.exports = function (grunt) {
           'src/helpers.js',
           'src/converter.js',
           'src/subParsers/*.js',
+          'src/subParsers/makeMarkdown/*.js',
           'src/loader.js'
         ],
         dest: 'dist/<%= pkg.name %>.js'