Standardize reaction receiving and posting

[rianadon]

Hubot's ability to write cross-chat-platform chatbots is extremely useful and amazing.

However, it lacks a standardized way of listening for reactions to messages or reacting with messages, as many chat platforms support.

It would be great if there was a standardized Robot.react method like the one both Slack and Discord support, as well as a method (something like Response.addReaction?) to post reactions to messages.

That way it would take minimal effort to port bots utilizing reactions between platforms.

Thanks!

[struppinet]

This would be really nice as also our Mattermost supports it.