Add usercmd to run after dispatch

[Kamilcuk]

This allows users to generically override dispatch with their custom behavior. Some kind of notification is required when the dispatch finishes.

For example, I wanted quickfix window to always open. I have this in my vimrc:

	function KcDispatchComplete() abort
		Copen
	        " let the cursor stay where it is.
		normal G
		wincmd p
	endfunction
	autocmd User DispatchComplete call KcDispatchComplete()

Amazing plugin, thanks.

[tpope]

We already call doautocmd QuickFixCmdPost make in this case. I've been reticent to add additional hooks for cases that adherence to the Vim standard already covers.

In terms of the problem you're trying to solve, I'm not sure this is the best approach. :Copen itself kicks off other autocommands, I feel like you're setting yourself up for jankiness. You can also force the window to open by changing the exit status of the command to nonzero or the 'errorformat' to match any line of output (e.g., add %+G%.%# to the end). I could see an argument for making this easier to do.

[tpope]

See also discussion in #339.