Sending on-demand emails requires a Mandrill API key, not a Mailchimp
one. (They're part of the same company/suite now, but still use
different keys and endpoints. Mandrill and Mailchimp keys are also
called "mailchimp" and "marketing" keys respectively. Yes, in that
order.)
Successfully sent an email like so:
```hoon
=vars `(map @t @t)`(malt ~[['name' 'john'] ['notifications' '1337']])
```
```
-bark!mailchimp-send-template "[KEY]" "tom@tlon.io" "landscape-weekly-digest" vars
```
TODO: handle the `vars` map
