Slash commands

[JasonEtco]

Why?

Adds /command support via a new #command method:

/* /deploy this-repo */
tools.command('deploy', arg => {
  console.log(args)
})

Closes #17

How?

Added a new #command method to the Toolkit class.

---

• Tests have been added/updated
• Documentation has been updated

[jerrylopez]

This would be pretty neat. I was just thinking about this over the weekend. I'm currently using Slack for /deploy but would love to do this from a PR and get rid of slack (i'm the only user on our slack workspace 😅).