Reactions

So I spent a bit of time some while ago working on adding reactions. This is all in the `with-reactions` branch of the repo. Keep in mind this is a much older branch and there have been a lot of changes since than on `master`, but I feel we can port our logic from there when we're ready.

This is what I had working:


You'll notice Slack inspiration there :P

An important thing to point out is that the [Reactions APIs](https://developer.github.com/v3/reactions/) are still in development preview. Definitely worth discussing if we wouldn't mind including it now and changing it in the future _or_ if that's not the safest option just yet.

Looking forward to having this rolled out eventually. Discussions/contributions are always welcome :)