Support pinned (a.k.a. static) bundles

I have revived this old piece of code that was sitting on my fork, which only makes sense to integrate now that the installer remembers the configuration.

New supported option called `static` for the Bundle command. When set to 1, the bundle is added to rtp but no install/upgrade operation is performed.

Fixes #24, #397

@gmarik @starcraftman Thoughts?