Add image dimension support for template languages like Slim

This patch allows support for templating languages like [Slim](http://slim-lang.com/) and [Jade](http://jade-lang.com/) which don't have the angle brackets on img tags.

When using AutoFileName, this patch will allow turn templates structured like this:

          table
            tr
              td width='50px'
                img src="images/my-image.jpg"

into this:

          table
            tr
              td width='50px'
                img src="images/my-image.jpg" width="50" height="50"

Regular HTML `<img ../>` tags will still work, as some templates support them.

To enable, the template scope must be added to `afn_valid_scopes` and `afn_template_languages` must be set to true, like so:

    ...
	"afn_valid_scopes":["string","css","sass","less","scss","slim"],
	"afn_template_languages": true,
    ....