Add support for TOML files

[akaIDIOT]

TOML as a file format needs little introduction. I think confidence could and should support the file format in a way, as it's becoming the default format for any kind of configuration in the Python ecosystem. The question is maybe: how exactly...

• should the default remain YAML?
• should entry points like load_name look for .toml (at least the spec states it should always be this extension, nice) files by default?
• can YAML and TOML files be mixed?
• what should a manual switch between the two formats look like?
• should tomli be an optional dependency or hidden behind an extra?
• should pyyaml become an optional dep?

There's other advantages, like the format stating that a dotted key should be parsed as a nested dict, exactly what confidence will do internally with dotted keys read from YAML. It doesn't seem to support any kind of referencing natively, so that remains something where confidence would be able to add value.

Interested in ideas from other users on this 🙏