Use ~/.config / XDG_CONFIG_HOME as config file folder

[cweiske]

Instead of polluting my home directory, the hamster-bridge configuration file should go into the user's configuration directory.

See https://specifications.freedesktop.org/basedir-spec/latest/ for the specification.

[elbenfreund]

hamster uses a beefed up version of appdirs in order to try handle this a clean and platform agnostic way. Unfortunatly is looks like the original appdirs repository is unmaintained by now, so we considered publishing our own stand alone fork before it seemed a bit too much work.