Add force option to cleanCache method

[javierbrea]

As a cleanCacheThrottle option was recently added (issue #140), it is desirable to add a force option to the cleanCache method, for those scenarios in which the throttle should be ignored because the cache has to be cleaned instantly.

The method could accept options as an object, so the forced invocation could look like:

provider.cleanCache({ force: true });

If the cache is forced to be cleaned, the throttle time should be reseted and any pending invocation should be cancelled.