Semaphore Build Status Webhook

This is a simple plugin to parse an incoming build status update from the default Semaphore webhook, turn it into something that's Slack compatible, and send it to a channel you have configured. A few notes about this PR:

* Currently in my team's slack I am not seeing the icon that I have chosen for this bot. I do, however, see it in my own Hammock interface when I look at this integration. Not sure if that's a known issue in Slack at the moment, or something I've done wrong in this plugin.
* While I'm sure they'd provide permission, I have not asked Semaphore for permission to include their logo here, nor did they provide these logo files. I just grabbed their Twitter Avatar and Photoshopped it a bit to make a transparent PNG of it.

I look forward to any feedback or other suggestions you might have to improve this. :metal: