add hook to allow play-json users to provide their own Jackson JsonFactory or ObjectMapper

[pjfanning]

play-json is pretty dependent on Jackson for low level parsing support.

There are some open but not progressed PRs and issues regarding supporting Jackson features. Allowing users to provide their own JsonFactory or ObjectMapper would make it so that play-json users could uptake some of these Jackson features themselves.

Doing this at the JsonFactory level would be better because of the way that Jackson is evolving.

Relates to:

• refactor Jackson code to hardcode buffer-recycler #999
• support jackson-core fast number parsing #835

[mkurz]

fyi, while working on playframework/playframework#12662 I added a private[play] method to set the objectmapper.. Not sure if this is needed by others as well, however at least from a Play application the play-json objectmapper can now be configured.
In case we want to provide this functionality in play-json as well I think JacksonJson needs a bit more refactoring.