CalVer: pattern mode support for formatted commit time

As we discuss using this plugin many are asking if CalVer can be an option. There are a few ways this could be interpreted. One is to allow any timestamp to be requested in a specific format:

`${HEAD_COMMIT_DATETIME:yyMMdd}`

I personally don't think of this as CalVer (all cal, no ver) - but it _is_ using the date in the version so meets the entry requirements and it is a useful adornment to the version for frequent build/deploy pipelines!

https://calver.org/
https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html#patterns