We should consider having another user-scoped config.

Currently we have `.ko.yaml` as a project-scoped configuration file and `KO_DOCKER_REPO` as user-scoped configuration, but it would be nice (especially when context switching projects) to be able to configure user-specific `ko` settings in some way.

The idea here would be to support `.ko.user.yaml` or something that could be put into `.gitignore` and would include a search path similar to #172, but also include the user's home directory. I believe that this can be done in a way that it complements `KO_DOCKER_REPO` (considering the existence of `viper.SetEnvPrefix`).

Start by reviewing the existing .ko.yaml and KO_DOCKER_REPO configuration, then read issue #172 for the proposed search-path behavior. Define how a .ko.user.yaml file and the user's home directory should be searched, how it complements the environment variable, and what configuration precedence should be documented and tested.

Written by the indexing model from the issue text.