Feature Request: support :skip_reference_warnings_for

[eksperimental]

I think it will be useful to have an option: skip_undefined_reference_warnings_for:
if a function is listed here and it is referenced to, the warning will be ignored.

It addresses #1151 (comment)
where calling ExUnit.Case.__using__/1, which is a macro with @doc false and it will emit a warning when referencing it.

I can submit a PR if agreed on adding this feature.

[josevalim]

Sounds good to me too!

[eksperimental]

I already have a working version for this, but I'm waiting for #1233 to be merged