Add setting versionPolicyModuleVersionExtractor

[jgogstad]

Add setting that lets users inject parsing logic for module revision to version numbers. This will work as an escape hatch for situations where dependencies have non-standard version schemes. See linked issue for discussion.

This PR is reviewable on its own, so not making any attempts to fix the dynver custom separator issue now. Reviewers should have in mind whether this new setting could be a good fit for that issue too though. I don't think it makes sense to consider this setting for versionPolicyPreviousArtifacts or versionPolicyIgnored, but please give feedback on this as well.

fixes #139

[julienrf]

Thank you @jgogstad for opening the PR!

The changes look good to me. I have one comment about the choice of using a total function vs a partial function.

Also, could you please add documentation (in the README) about the new key, and its specification?

[julienrf]

Thank you @jgogstad for addressing my comments. After thinking more about it I found some issues with the current implementation, please read my new comments below.

[jgogstad]

I was bit unsure where you want the default value for versionPolicyModuleVersionExtractor to be bound, please see previous commit.

Thanks for the thorough review and sorry about forgetting to update the test case

[julienrf]

Thank you @jgogstad, this looks great to me!

[julienrf]

Just a couple of things before we merge the PR. Could you please:

• Add a section about this setting in the README? (at the end, before the acknowledgements section)
• Squash all the commits into a single one?

[jgogstad]

Thanks, done. I kept the original description for the settingKey, it defaults to _.revision through applyOrElse now, not by a default binding. I think it's more informative to keep it as-is.

[julienrf]

Thank you!