WiP: Added PEP518 support

[carstencodes]

The PIP518 support as described in https://github.com/PyCQA/pycodestyle/issues/813 has been implemented roughly.

It uses the toml package, since the toml package describes itself as being Python 2 compatible. Using another package will create a dependency that might not be wanted. Hence the dependency is treated optional as an extra, so consumers can decide which revision they would like to use.

TODO: Unit test must be written explicitely.