Add attributes support to generated script tags

This pulls adds a expanded format for `src_files` and `serve_files` that allows attributes to be added the generated script tags when not using a custom test page.

There are many use cases for this eg:
* requirejs' `data-main` attributes
* blanketjs `data-cover-only` (among all it's other config)

This does not introduce a BC break, simple adds an extended format.