Description
Describe the feature or problem you’d like to solve
Configuration can only be specified in a single location (typically $HOME/.config/gh/{config,hosts}.yml on *nix systems). This makes it hard for system administrators to provide default configurations for their users, unless they autogenerate the user's home config files. But in that case, users can not easily add custom configuration (for instance, their preferred editor).
Proposed solution
Allow a cascade and merge in which a set of system- or user-specified config paths are searched and loaded, with later configuration overwriting previous ones. This would allow, for instance, system administrators to put default configuration in something like /etc/gh/config.yml while still allowing users to override as necessary.
Additional context
This is split off from #4081, and would also improve the experience of the configuration value proposed in #4249 (though this issue and that are technically orthogonal).