Pass feature to the switchable method

[tgriesser]

Thought this could be useful if you want to disable the toggle for an individual feature, useful mainly with custom strategies.

[rolftimmermans]

Agree this is useful. Unfortunately it is a backwards incompatible change so we'd have to bump the major version. Would there be a way to make it backwards compatible? Maybe by checking the arity of the strategy and implementing a fallback mechanism for the old API?