Add ObjectMapperFactory#configure

Provides a means to configure existing `ObjectMapper` instances using the Dropwizard `ObjectMapperFactory`.

Breaks out the code to configure instances from `ObjectMapperFactory#build()` in to `ObjectMapperFactory#configure()`.

This is also useful for sub-classes that override `ObjectMapper#build()` to create a sub-classed instance of `ObjectMapper`, but configured according to the `ObjectMapperFactory`.