feature: support preserving focus with on_result_diagnostics_quickfix

[jpetrie]

Using on_result_diagnostics_quickfix with open set to true causes the quickfix list to open and take focus, which can be distracting. I'd like to be able to configure Overseer to keep (or specifically, restore) the focus to the previous window once the quickfix list is opened. I don't see a way to do this currently, but digging around the source it looks like the desired behavior is obtainable simply by appending " | wincmd p" to the command used to open the list.

Plumbing this through properly (with an option such a keep_focus that defaults to false to preserve existing behavior) doesn't seem that involved, so if this is a feature that seems acceptable I'd be happy to send a PR for it. If on the other hand there's a way to do this already that I've missed, please let me know.

[stevearc]

Adding another parameter to control this sounds reasonable. It may be more complicated than just adding wincmd p because if the quickfix is already open prior to that command, we don't want to switch back to the previous window. Might make more sense to just store current window and restore it via lua API

[jpetrie]

I realized the same thing when I went to implement it; the PR I submitted in #237 does exactly that (the same way on_output_quickfix works, basically).

[jpetrie]

#237 was merged and addresses this. Thank you!