Pass request object to limiter callbacks

[dlindenkreuz]

The request object cannot be accessed easily in some of the limiter callbacks (limit_value, exempt_when). Support for this would be really useful. This already seems to work with key_func which gets passed the request if the callback declares a request parameter.

This would also remove the need for the workaround from #13.

[Kavan72]

@laurentS do you have any plan to add this in near future ?

[fredi-python]

Are there any news to that?