Expose tag selector function on Reckon builder and extension

[michelzanini]

The use case here is to be able to recognize other non-standard tags, as for example master-RELEASE-1.0.0, or RELEASE-1.0.0, etc...

This is just a preview to start a discussion, maybe you would like this to be done is some other way.

I have made the simplest possible solution to expose tagSelector to the builder and to the Gradle extension. I understand maybe you don't wish to expose it to the Gradle extension.

There is two other ways I can see a solution for this problem:

• We could instead expose a regex String, and adapt the code of GitInventorySupplier to work with a regex pattern

• We could just not expose anything, but write a more inclusive default tag selector. For instance, we could instead of removing just a v, remove any non number string from the start of the String until we find the first number...

Let me know what solution you see more fit, and I can change this PR.

Note: I have not looked at the tests yet as I want to wait until you approve the approach before doing that (but I made sure the test still pass).

Thanks.

[ajoberstar]

Sorry for the delay. Superseded by #171.