Add an option to `cog push` to update the image in `cog.yaml`

[bfirsh]

Perhaps cog push -u ... should work, like git push -u. That way we can give user a copy & paste-able thing to set everything up on Replicate.

The downside of this is that we edit a user's file they might have in source control, which is also unexpected. Also, writing YAML programmatically is usually a really bad idea, but maybe it's fine if we're just adding/update a single line. (We can use a regex instead of trying to write out the YAML file again, maybe....)