add option to lint diff buffers

[blobmasterbrian]

Adds an option to lint diff buffers as suggested below:
#2583

Tests added, documentation updated.

[Hnasar]

Looks good! Thanks for the patch. We'll need a maintainer to review though.

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[blobmasterbrian]

@w0rp tests have been written and documentation added. Can we reopen this and get it merged?

[blobmasterbrian]

@Hnasar @w0rp can we add this functionality?

[blobmasterbrian]

@w0rp @Hnasar after using vim-lsp as a standalone for a few years, I'm coming back to using ALE precisely because the error messaging in vim-lsp in diff buffers caused odd jumping around the file. I hadn't realized this was never merged because I had it locally, but if I'm coming back because vim-lsp is insufficient for diff buffers then it doesn't really make sense if it's not supported. Can we reopen and merge this?