Add a configuration system based on git-config

[abhinav]

git-spice needs a source of configuration to support customizing behaviors like submit navigation comments, custom aliases, etc. After some deliberation, we've decided to use git-config as the configuration source for git-spice.

This will allow for a familiar configuration experience with user-, repository-, and worktree-level overrides for options.

This PR does not implement any configuration options yet. Those will be added in subsequent PRs.

Resolves #16

[abhinav]

This change is part of the following stack:

• 326 ◀

  • 328

    • 327

_{Change managed by git-spice.}

[abhinav]

This is supposed to be user facing except there are no actual options yet, so no changelog entries to add.