Implement :execute command

`:execute` is important piece of VimScript and its usefulness can start to be realized now because of the increased expression support that I've been working on. Also it was a fairly low-hanging fruit given that it is just pumping stuff back through the same VimScript engine we are already building.

I have in my mind the distant goal of being able to support some simple plugins and in my experience `:execute` is very common. It also works in conjunction with the `.` operator I just implemented.