Support TOML

[marioortizmanero]

TOML is a very similar format to INI, but with types and a formal standard. Adding new argument parsers can be more complicated, but this shouldn't cause too many problems, as it can use the section attribute, too. In case I end up supporting more configuration methods I would like to follow more or less what's done at config-rs, which supports multiple config file formats and environmental variables by using feature.

I personally won't use TOML for my projects, so if I end up supporting TOML myself it'll be in some time, or when I find myself needing the features it provides. Anyone is free to contribute anything :)