Introduce support of add and del operations under erl_opts directive

[juise]

No description provided.

[ferd]

Can you provide the rationale for this change so that we can review it with the proper context?

[juise]

Yeah, sure. We have rebar.config, with directive erl_opts which contains opts like 'warn_missing_spec', which is necessary and helpful on compile, but is very annoying when tests are run. Just override erl_opts in test profile is not correst, in erl_opts may be useful opts like pars_transform. So, we need the way to remove some options from erl_opts directive.

Additionally I've do this for deps too, this will allow to keep 'override' option consistancy.

[fenollp]

Did you try adding nowarn_missing_spec to the test profile?

[juise]

@fenollp yeah, this doesn't work when you have dependencies, with warn_missing_spec flag in opts. But it's works for single app w/o deps.

[ferd]

After looking at it, one tricky aspect of it is that I think this will die when trying to delete an option that is not a list. For example, {minimum_otp_vsn, "17.4"} cannot be deleted in any way with this option, only overridden. This option can't be added either since add only works for lists so I guess it's fine.

Thanks for providing tests as well!

I'll be discussing this a bit with Tristan with regards to its inclusion.

[ferd]

Alright. Under the light of recent issues with things like warn_as_errors and other irreversible options, we consider this to be a good idea to introduce to rebar3 as a feature :)

Thanks for the contribution!

[juise]

Awesome, thanks