Add multi-channels, status-specific channels, and branch-specific config

[ezrast]

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[ezrast]

Haha whoops, I meant to PR this to master on my own fork and do some more testing before sending it to you.

But anyway I added the following things to your thing:

• can specify multiple Slack channels with an array
• can override settings on a per-branch basis by turning branches_to_handle into a hash
• can override the default slack channel for specific build states by specifying a key like "channel-"

I don't think it breaks configuration on any existing deployments but like I said I haven't exactly tested it extensively yet.

Merge if you wanna, don't if my changes suck.

Peace