[Fix #152] Add section helpers to all <h2 class="content-subhead"> elements

This pull request fixes #152 by adding a new Handlebars Helper called `anchor`.

## Usage
Using `{{anchor "My title"}}` will return `<h2 id="my-title" class="content-subhead">My title</h2>`. You can override the element type and pass in a list of classes as well:

`{{anchor "My title" "h3" "myclass}}` will render out `<h3 id="my-title" class="myclass">My title</h3>`. The regex parses out HTML elements and other special characters from the title to make sure the `id` attribute doesn't look too funny.

I went ahead and replaced all the `<h2 class="content-subhead">` with this helper.