Previously, failing other checks, on Unix and BSD systems,
`config.Paths` would attempt to compute the user's home directory by
reading the `HOME` environment variable.
This change deprecates that approach with a call to `homedir.Dir`, which
is used elsewhere throughout the application.
Attempts to resolve an issue regarding automatic config file generation,
as referenced in #501. This issue is believed occur when
`XDG_CONFIG_HOME` is unset.
Performs a minor refactoring on `config.Paths` to consistently use
`path.Join` when computing config directory paths. Previously, both
`path.Join` and `fmt.Sprintf` were being used, strictly due to an
oversight.
