Allow loading `resource_owner` to be configurable.

[doomspork]

Right now ExOauth2Provider relies on preload/2 to load the owner from the access token: https://github.com/danschultzer/ex_oauth2_provider/blob/master/lib/ex_oauth2_provider.ex#L92-L98

In our application this less than ideal as we need to do some additional loading alongside our user and now we're forced to maintain that logic in 2 separate parts of the app.

Would you be open to a PR that made this function optionally configurable or overrideable?

[denic]

Second that. Also I would like to add a caching layer and not hit the DB on every request.

Would be nice if we could just add a configuration option for the module that should be used to resolve the resource owner.