Add support for pyproject.toml for configuration

[sgraaf]

Add support for configuration using pyproject.toml, as specified in PEP 518.

Inspired by both isort and mypy, I have resorted to the tomli package for parsing TOML files. As such, this PR introduces the first non-dev dependency to darglint.

Closes #130.

[terrencepreilly]

Sorry for the late response; I've been handling an estate for a death in the family, and I haven't been very active with this project lately, anyway.

I'd prefer not to add any dependencies to darglint, if possible. It looks like a TOML parser will be added to the standard library in the near future, though. So, as long as the configuration is relatively simple and won't require many changes to adopt the standard library version, then I suppose it should be fine.

The tests are currently failing, though. It looks like the dependency isn't correctly installed when running the tox script. Once that's resolved, it should be good to merge.

[raddessi]

I love the concept of making tomli optional so this feature could be added. Having a single config file makes the top level repo directories so much cleaner. Thank you for the work!

[ghost]

Any updates on this?

[theNicelander]

Any news on this one? Love this tool and it would make it even better to be able to define the config as part of poetry :)

[ssbarnea]

@sgraaf @terrencepreilly Any chance to revive this?