Expose key docstring metadata

[ryan-summers]

As part of the derive macro, it would be useful to use a Key to look up the docstring related to the entry. For example:

#[derive(miniconf::Tree)]
struct Settings {
    /// The selected sampling rate in Hz.
    sample_rate: f32,
}

assert_eq!(Settings::get_metadata("/sample_rate", "The selected sampling rate in Hz."),

This would allow the code comments to be exposed as helpful information about the settings tree