- Most of this change is documentation improvement for the `Config`
  class and some of its methods.
- Calling `Config.snapshot()` previously raised an attribute error if
  the `Config` did not come from a repository (i.e. the system or global
config or a config loaded from a file). This is fixed by ensuring that
`self._repo` is always set to something (`None` by default).
- Passing a `PathLike` into `Config.__init__()` resulted in a value
  error unneccesarily. This is supported now.
- Added tests that confirm this behavior.