Recognize how slim sometimes outputs safe html (with `escape_html_safe`)

Reduces slim XSS false positives in cases where brakeman is run in the context
of an app that includes active support.

Templates compile down to Template::Utils.escape_html_utils if
Active Support's `html_safe?` method is defined.

See:

https://github.com/judofyr/temple/blob/master/lib/temple/filters/escapable.rb#L12
Merge pull request #410 from noahd1/reduce_slim_false_positives

Reduce Slim-template XSS false positives