Auto-discovery of build definitions from rbenv plugins

The `share/ruby-build/` directory from each rbenv plugin, if it exists, is added to RUBY_BUILD_DEFINITIONS automatically during `rbenv install`.

@sstephenson: I first started adding this functionality to rbenv itself, but I figured I could just as well add it here and keep rbenv blissfully unaware of ruby-build and its concerns. Thoughts?

Continuation of #613, #608 /cc @parkr @thomasjo