Added ability to specify a custom persistence provider.

[davewthompson]

Can now add a custom persistence provider. Open to ideas

[adebisi-fa]

Hello @guyspr,

Please is there a technical reason why this request hasn't been merged with the base?

It's a very important update, very key to opening up bpmn to more usage options.

Thanks please.

[qingxiao]

The more I get to this problem when I use the BPMN module, it's a good change