Add support for select target callback

[xubury]

Although select_target does not return a plenary job, but it use vim.ui.select which can use callback on confirm. These commits add a after callback for that, so you can do something like select target and build

[Shatur]

I'm not sure I like this idea. This makes API more bloated. There is no issue in having two shortcuts, one for build and another to select a target. You will need then anyway.