Makes storage location configurable.

The default breaks on CI, so need to be configurable for all environments.
It's a small one, but I kind of want this property to explicitly default to the java temp directory. 

Firstly if for some reason the flapdoodle implementation changes or breaks, the behaviour of the plugin will remain consistent. Secondly I don't like passing around nulls :-) If I'm perusing the code I like to be reminded what this value defaults to, rather than having to check the docs.