Handlebars

This pull request adds support for indenting Handlebars expressions (http://handlebarsjs.com/) in the beautify-html js formatter.  It adds a new option and indents Handlebars block expressions, e.g. {{#if}} and {{/if}} at the same level as html tags like <div> and </div>.  Since there weren't any HTML formatting unit tests, I added some of those as well (although they're not very extensive) so that I could test Handlebars expressions too.

This change was useful for me, so I wanted to submit a pull request in case you were interested.