Add support for htmlbeautifier gem for fixing erb files

**Name:** htmlbeautifier
**URL:** https://github.com/threedaymonk/htmlbeautifier

This is the only tool I know of that properly formats `erb` files. I'm currently using it in my fork of ale, but I don't really know anything about vim script so it's just done simply to solve my problem. There aren't any tests or anything.

I can create a pull request, but it isn't ready to merge as is.