Support Dispatch to run asynchronously

[m1foley]

Since RuboCop is typically slow, it would be great to run asynchronously. An example of a plugin that does this is vim-rspec.

I tried implementing this myself but I don't understand Vim's compiler system. Here are some links that seem relevant:

• When running rubocop through Dispatch, pipe chars prevent opening files from the quichfix window tpope/vim-dispatch#103
• quickfix errorformat doesn't work for dispatch tpope/vim-dispatch#41