Dictknife

- fix bug `unexpected keyword argument loader` is occured when yaml library is not installed

- use tomlkit instead of qtoml
- use ruamel.yaml instead of pyyaml (for preserving comments)
- add `dictknife diff -o pair`

- loading, see envvar (DICTKNIFE_LOAD_FORMAT, DICTKNIFE_DUMP_FORMAT)
- omit oauth2client dependency, if spreadsheet's requires

- add dictknife.naming
- add builtin transform functions (snakecase_dict, camelcase_dict, kebabcase_dict, ...)
- rename function, dictknife.transform:normalize_dict -> dictknife.transform:str_dict

- add dictknife.accessing.Scope (108)
- fix: fix accessing with list (113)
- fix: fix maybe access (112)
- add 2to3, but not complete implementation (115)
- add jsonknife bundle, --flavour option (133)
- add jsonknife separate (137)
- use qtoml instead of toml (156)
- fix: use pyyaml's sort_keys option, instead of custom implementation (162)

- loading, adjustment yaml's output