Allow sending emails in reaction to any order transition

The module supports only the "place" transition by default, I think we can add support for all order transitions by defining a plugin deriver.

The attached patch adds support for any order transition (See the screenshot below):

Wasn't sure how to construct the label as using the transition label as the event plugin label doesn't sound like the right option...

This is great. THANKS!!

Committed!

Automatically closed - issue fixed for 2 weeks with no activity.