Discover plugins to be executed

This branch is for plugin discovery.

@thsutton, @tranma: What do you think about changing the libexec/git-vogue OR environment variable behavior here to a concatenation of both?

I presume the rationale for this is that there's no way to not use the system-wide plugins unless we have this behavior, but perhaps that could be a per-repo option of a different sort.

I only bring this up because I would have thought it should work the *other* way (concatenation) before I had a think about it and now I'm thinking this way is more correct.

My instinct though is that we should pick the more intuitive method and provide an option for the "correct, ideal" behaviour. It's always better to behave as the user expects, you can't change user models.