Move community site interaction into a plugin

This is a first crack at what we discussed briefly [here](https://github.com/sethvargo/stove/issues/18), born out of how perfectly Stove could fit in my workflow, but how there are some cookbooks I'm prevented from releasing to the community site.

Some new-to-me Ruby idioms in Stove, so any feedback is much appreciated. Or, if this is too much against its original purpose, that's cool too. Thanks!

Also fixes an issue where the GitHub plugin would try to release cookbooks to `https///github.com/...`.