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 
If you run `rbenv install --list`, this code is never executed, therefore `rbenv install --list` won't ever show custom plugin-based definitions.

If you move this code up before line 69, it'll work.