Support `pyproject.toml` and `setup.cfg` for features operating on `tox.ini`

[The-Compiler]

Problem

According to the tox configuration specification:

At the moment tox supports three configuration locations prioritized in the following order:

1. pyproject.toml,
2. tox.ini,
3. setup.cfg.

But PR's such as #10 only consider tox.ini. Note that the syntax in the other files is somewhat different...