Feature request: add ability to trigger the bot

[petrgazarov]

Hey there,

We've been using seal for some time and it helped us in our managing our git-workflow. One feature that we found useful (and which I implemented in a fork) is being able to trigger the bot.

This can be done fairly easily with slack-ruby-bot gem that does all the web socket and Real time messaging heavy lifting. Would you be interested to add this functionality? If so, I would be happy to submit a PR.