The logic for deciding start & next version number should be externalized

[stigkj]

Add an option to the plugin that holds a closure which will be called when needing the next version number.

This means expanding the plugin's extension object with this option that by default holds a closure that has the same logic as what is done today:

nextVersionNumber = currentVersionNumber + 1

[stigkj]

Also needs an option with a closure that defines how the first release tag should be calculated.