Add :CommandTLoad command

This command is intended to make extending Command-T via "monkey
patching" easier, by providing an easy way to trigger immediate loading
of the plug-in files, rather than relying on prior lazy loading, or
having to replicate the `LOAD_PATH`-management logic that
`autoload/commandt.vim` provides.

See motivating discussion at:

https://github.com/wincent/command-t/pull/131