get value from data attribute

In support of [this story](https://www.pivotaltracker.com/story/show/82594664), this PR adds the ability to specify the "value" attribute of an autotag request in a "'autotag-value" data attribute on the element.

This will be useful for example in distinguishing between HomeBuilder and Community lead requests in NHG.

This PR also includes some "administrative" cleanup. The gulpfile.coffee file was removed on @lfender6445 's advice, since it is incomplete.
What do you think about moving the `autotag-` prefix into the main `opts` (and this be the default). It would be great to communicate how to use data attributes. For instance, future changes can support `data-autotag-sg`. `data-autotag-*` is the standard to work from.

Updating the README would be helpful as well.