JSON, TOML, YAML: support for nested config data

[peterbourgon]

This PR is a simple adaptation of the TraverseMap function from jolheiser#1 (thanks, @jolheiser!) which is applied to the JSON, TOML, and YAML config file parsers more or less in the same way. It allows nested config data in any of those formats to map to flag names in a reasonably well-defined way.

Addresses #107 and #108. I'm pretty sure this isn't a breaking change, but I'd appreciate a review from anyone who'd care to give it a look.

edit: I broke fftoml for sure, I'll fix that.

This is a precursor to a much larger refactor, including many highly requested features, coming soon to theaters near you.

[lmittmann]

If you decide to introduce a breaking change at some point I would suggest dropping the type ParseError struct for something like:

var ParseError = errors.New("error parsing TOML config")

// ...

func (c ConfigFileParser) Parse(...) {
    // ...
    if err := toml.NewDecoder(r).Decode(&m); err != nil {
        return fmt.Errorf("%w: %w", ParseError, err)
    }
    // ...

You would also need to upgrade to go1.20 for this change.

[peterbourgon]

It was a mistake to introduce error types to this package. The next version will absolutely remove them.

[peterbourgon]

Thanks for your review.