Allow delegating to a Vim command in default dispatch

Not convinced this is anything but trouble, but the idea is that
sometimes you'd rather let another plugin handle the :Dispatch details
(e.g., rails.vim's :Rake) or just do something else entirely (e.g.,
scriptease.vim's :Runtime).  This works with :FocusDispatch too, but
note that :FocusDispatch :Rake runs :Rake literally, which is probably
not what you want if you switch buffers.