Support ignore for ignored variable used

[cypherfox]

This change adds the ability to add a list of ignored modules to the rule ignored_variable_used.

This is needed as elvis will fail on an ignored variable in a eunit ?assertMatch macro.

    ?assertMatch({ok, {a_valid_message, _V}}, fut(Args))

Not ignoring the variable will cause the compiler to report a warning instead.

[elbrujohalcon]

I think the idea is useful, but what if you fully ignore the variable (i.e. using _)?

To expand on this, be aware that if you use _V in two consecutive ?assertMatch's, for the second one to work, _V will probably (haven't reviewed eunit macros intensively lately) need to have the same value in both. Which is exactly what this linter rule is trying to prevent.

[elbrujohalcon]

Would you mind adding / adjusting some tests for this functionality?

[cypherfox]

Ah! Fully ignoring works. While I dislike it from a standpoint of literate programming (IMHO it reduces the readability of the test), that explains why nobody else has been asking for it for a long time.

Yes, I will try to add some tests.

[elbrujohalcon]

Thanks @cypherfox ! Don't forget to also update the wiki with this change

[cypherfox]

@elbrujohalcon done.