Ability to respond to emoji added to message

[mcMickJuice]

I'd love to have the ability to respond to the event of when an emoji is added to a message.

Use case: Our bot monitors our support rooms. We have users who post into our support who sometimes don't provide enough context or information for us to support them. Instead of us manually responding: "Please provide more information" it'd be nice to append the :question: emoji on the message (or something similar) and have our bot create a thread with the message and useful links

Dependent on https://github.com/target/flottbot/issues/23