Add support for handlebars script templates

This pull request brings support for "x-handlebars-templates" inside a script tag.
This is related to [issue #6 of this project](https://github.com/nono/vim-handlebars/issues/6) where people asked to support this.
While this update doesn't make it work in files with `html` file type, it enables people to use the "tweak" described in that issue.

Example is updated to show what is now possible.