PROPOSAL: Support sending publication information to a webhook

[notmessenger]

Builds on #125 except that a user can configure a web hook url that pr-bumper will POST the collected information to.

[job13er]

Works for me, I'd add both as a feature that can be configured in .pr-bumper.json, then you could specify the filename where to write the info, and the webhook to post to. The webhook part may need to be a little smarter though, I'd assume there should be some kind of authentication that we wouldn't want to live in .pr-bumper.json in the repo. That part will be tricky, since PRs from forks in github don't have access to private environment variables in Travis. If using some private CI though, it'd be useful.