Add FormKeep to resources as Jekyll form backend

Launched by [thoughtbot] today.
Intended to be Jekyll-friendly with
no iframes, JavaScript embeds, or CSS overrides.
It just generates a URL to use as your form's `action` endpoint.

It has a very simple webhook system, which means
it can automatically forward all submissions to a webhook of your choosing.
We've been using [Zapier] to handle the heavy lifting of
sending form data along from FormKeep to MailChimp, Trello, etc.

[thoughtbot]: https://thoughtbot.com
[Zapier]: http://zapier.com
Merge pull request #3010 from croaky/master