Implemented a very basic plugin system.

The way this works is pretty much the same way as `git` commands work.

When invoking `bundler <task>`, if the task isn't defined on Bundler::CLI,
Bundler will try and figure out if there is a `bundler-<task>` binary present
on the path. If so, it's invoked.

Same error message (and error codes) apply when a binary isn't found.

Also, this is just a first, simple pass at the problem. @indirect and I had a
chat about how it should work, and we should probably think about offering the
possibility of loading up Ruby plugins in the already loaded Bundler
environment.

(Ref bundler/bundler-features#8)