#fix 1060 Add 'con' to featuring artists markers

Thanks for working to unify our definition of a "featuring" token!

It seems like having a function in the `util` module might be a little bit brittle, though—it's an odd place for something so beets-specific. One possible alternative would be to make a global configuration option that's just one big regex (or two), which would make it obvious how to change this in the future. Does that make sense? If not, we can just keep the new function.