Evergeen functionality for messages that should stick around

[bruntonspall]

Inspired by https://github.com/18F/hubot-slack-github-issues#hubot-slack-github-issues

When someone says something monumentally useful in the slack, adding the evergreen emoji as a reaction should cause the bot to take the text and.... put it somewhere.

That somewhere could be:
A wiki or service manual, like 18F do
The bot's database, to create it's own knowledge base
The topic?