Add MGit Configuration.

* New AppDataVersion1. Files go to XDG config dir 'mgit', repositories
  file is called 'mgit/repositories.yml', added new config file
  'mgit/config.yml'.
* Make sure the migration works.
* Add new Configuration class with global mgit configuration options.
* Add first configuration option 'threads' which toggles the fetch command
  threading behaviour.
* Add new 'config' command to set config options conveniently.