"Up to date" column in `crane status`

Spin-off from https://github.com/michaelsauter/crane/pull/58, with a more appropriate (?) implementation for that particular use case. Basically a way to see if the base image of an existing container hasn't changed since it was started. In the end, I am not quite sure it has a strong value. At least not for me, since I never use `crane provision` nor use containers built via crane. But maybe some people do?

Also, the direction I took with the `status` refactoring is questionable, but I think I find it slightly more readable, in its own way. A matter of taste I guess...