vim8 async support

[dagon666]

A support for vim8 asynchronous API would be very much appreciated.

[djmadeira]

This is already supported; use g:ack_use_dispatch = 1 (you'll need to install vim-dispatch first)

[ches]

The Dispatch support has shortcomings—for one, there's no callback mechanism when a job has completed so we pop up the quickfix list early before results are ready to populate it.

I'd be glad to see support for the Vim 8 jobs API. I'm disappointed that Vim and Neovim went in different directions on this, maybe something like this could help support both.

[sj26]

I gave this a go using pure vim 8 job control: #247