Expose Settings in CLI

• Use pyxdg to create default locations for state files (the various repository components) for a user setup.
• Create the Settings derivatives UserSettings and SystemSettings to provide defaults for a user and system mode of the CLI.
• Create an instance of UserSettings or SystemSettings when calling repod-file (depending on ArgumentParser).
• Create directories required for repositories (and only fail if this is not possible due to permissions)
• Extend repod-file subcommands with the ability of indicating the target repository to import to/ export from
• Adapt documentation for configuration file changes (e.g. user/system mode, overrides, configuration options) by adding a dedicated man page for repod.conf

Fixes #75 (closed)