Add mailchimp support

First attempt at adding MailChimp support

This works, but there are some issues that make it less preferable than simply using the form provided by MailChimp.

- The form submission takes you to a MailChimp hosted signup page and there's no (apparent) way to get an async response.
- There are several pieces of information that are needed to interface with MailChimp rather than just a simple API key. The way I approached this was to put them all in the `key` parameter and use that as the form action. It works, but it's not exactly intuitive compared to how the other services work.
- The name for the email field varies per list, and isn't necessarily consistent like it tends to be with the other services. Field names can be customized by overriding the markup, as with other services, but it's more likely to be necessary.  It also makes using this module effectively pointless, because all the module is doing is filling in the form's action attribute.

@pushred this definitely needs some work in order to be practical/worthwhile. What are your thoughts on what direction we should take this from here?