offset find should allow specifying which track to test

[eharris]

It would be nice to be able to specify which track to perform offset testing on, as sometimes the first track might be damaged.

Also, why is it necessary for every track to be successfully verified before whipper decides to use a particular offset? I often find that particular CD's, even when they are in good condition, have one or more tracks that might fail to verify due to a scratch or physical defect. Surely if more than half of the tracks are successful matches, that's enough to know that is the proper offset?