Take config out of package

[aleksandervines]

Well, at least add option to have it in a "user-controlled" location.

E.g. on unix/linux have a .pythesintrc.yaml file in home directory that is checked before loading default config from resource_string(__name__, 'pythesintrc.yaml')

[aleksandervines]

well, should use xdg in the same way as we used for json files for this.