Seeking a consumer by offset

Heya,

I have a need for seeking a consumer, i.e. the equivalent of reopening a consumer with the desired offset. Simply setting the @offset variable in the current implementation would work just fine, I believe [1]. Will you take a patch for this?

[1] More than believing, if I do "consumer.instance_variable_set(:@offset, N)" , everything works as expected :)   
