Detect and warn about incorrectly used ignore-comments

[RKushnir]

This PR addresses the issue of accidentally ignoring too much code coverage. The issue happens when you add coveralls-ignore-start but forget to add the corresponding -ignore-stop. With this change, excoveralls is going to discard the un-closed ignore-block (i.e. not ignore the coverage) and give you a warning, so that you can more easily spot the mistake.

Besides the main goal, this change also adds a few other useful warnings to let you know about redundant or invalid ignore-comments, which I previously outlined. This is an optional part, and we could remove it or split it into a separate PR if that would be more appropriate.

The most interesting part for review is ignore.ex and ignore_test.exs.

Here's what the warnings look like in the console output

The PR is unfinished, it's missing some of the final touches: some tests, changelog entry, version bump, readme update. But it's feature-complete and so I would love to get early feedback, in case I need to change anything before I polish it. I'm looking forward to your thoughts and suggestions.

Resolves #197.

[parroty]

Thank you for the PR 🙇 . I think it makes sense to emit warning for avoiding unexpected behavior. It would be beneficial as long as existing behavior is maintained (just providing warning and doesn't throw error?)

[RKushnir]

@parroty Thanks for the feedback. What about the coverage? Should we ignore coverage if there is only ignore-start without ignore-stop? (existing behavior)

[parroty]

@RKushnir I believe it's already a large change, so I am wondering taking a reasonably small step (while maintaining existing behavior) would be an option? (rather than extending coverage?), but looking for your opinion, too.

[RKushnir]

@parroty I agree. I think it's already a good progress if we just show the warnings. I'll try to update the PR soon.

Later we can add some config option e.g. --warnings-as-errors to fail the build if there are any warnings.

[RKushnir]

@parroty I updated the PR: reverted the behavior change as we discussed, added more tests and a changelog entry. Unless I missed something, it should be ready now. Thank you for your help.

[parroty]

Thanks and very sorry about taking time to merge. I'll try merge and release during this week 🙇 .