Additional HTML container groups

First of all, thanks for making this plugin. I use it for working with `handlebars` and it works nice!

About this pull request: I've noticed that handlebars (or mustache) variables `{{ ... }}` aren't highlighted properly when inside `<a href....>{{ stuf }}</a>' tag. To be precise - they are highlighted as text within a html link (special + underline highlight).

The same happens within `<u>` and `<b>` and `<i>` tags.

From looking at the plugin source I got the impression that mustache/handlebars variables should have the highest syntax highlight priority. (at least they seem to have more priority than html header tags)

Hopefully, this pull request enables proper highlighting for handle./mustac. variables within `a`, `u`, `b` and `i` tags.

Let me know what you think?