Ale: Support opening the loclist window only when files are saved

[amadeus]

Thanks a bunch for adding the loclist/quickfix population in a recent PR!

One feature I would like, if possible - the option to ONLY show quickfix on save. I'd still like the signs to show up in 'real time', but I would prefer not having the loclist/quickfix pop up while typing.

I am ok with it updating in real time once open however.

[w0rp]

This has been implemented now. See :help g:ale_open_list. You can use let g:ale_open_list = 'on_save'. Also, b:ale_open_list and b:ale_keep_list_window_open can be used now.