Document View::command and View::preempt

People in atom/vim-mode had questions about these methods but they weren't doc'd. I think we should doc them because they are helpful for packages like vim-mode. Exposing View::preempt is a little scary, but I  think it is worth it.

I delegate through to the jquery-extension methods because we might use those methods in non-view objects.

/via atom/atom#1620