Customisable ConfigReader instances for Java time

[vlovgr]

Ciris currently has ConfigReader instances for many java.time types in the core library (see JavaTimeConfigReaders). Some of these types support parsing with different formats, rather than the default one used by the current instances. The idea is to add configurable java.time instances, where the user supplies the format (DateTimeFormatter) and we create a ConfigReader for the requested type.

For inspiration, PureConfig has done something similar, as seen here:
https://github.com/pureconfig/pureconfig/blob/master/core/src/main/scala/pureconfig/configurable/package.scala

[fmsbeekmans]

Started working on this.