Add "Ignore But Retain" plugin

This incorporates changes from https://github.com/mwhite/Vellum/pull/60, https://github.com/mwhite/Vellum/pull/61, and https://github.com/mwhite/Vellum/pull/62.

In order to avoid the version control overhead of having every vellum include listed in the form_designer.html template in CommCare HQ, and having to duplicate those includes in tests/runner.html -- a significant disincentive to use the plugin system to add functionality to vellum in a modular, testable manner, as I've done here -- I added a grunt config to concat and minify all files into one, so there's a single point of truth for the vellum include files.

This goes against the general principle of not including generated files in version control, but seeing as it's minified, automated (although the git hooks integration isn't working at the moment), and at a module boundary, I think it's worth the benefits.

This can be followed by https://github.com/dimagi/commcare-hq/pull/2383