Integrate with reactiongifs.com

Introduces a new `reactionify` directive for agent99 that prompts her to grab a suitable GIF from reactiongifs.com.

E.g. `agent99 reactionify derp` might yield http://www.reactiongifs.com/wp-content/uploads/2013/07/dale-derpin.gif

This introduces a dependency on my industrial-strength wrapper around the reactiongifs.com "HTML API": https://github.com/harto/reactionifier

@richo do I need to handle exceptions in the plugin?