Volkanic

- require Python 3.6+

- remove `GlobalInterface.{_split_name,jinja2_env,_get_conf_search_paths}`
- add `environ.PathType`

- add GI.package_dir, GI.project_dir
- rename GlobalInterfaceTrial => GlobalInterfaceTribal
- python_requires: ">=3.5"
- unrequire pyyaml
- add GlobalInterface.namespaces
- add utils.under_home_dir_hidden()
- ErrorInfo.iter_related_files()
- utils.guess_content_type()

- remove GIMixinDirs
- CommandOptionDict: _expand, _explode

- remove CommandConf
- remove volkanic.default

- add utils.load_json5_file()
- add utils.discard_arguments()
- add GlobalInterface._options