safe get methods for Yaml

[blaenk]

It's nice that we have convenience Index impls for Yaml, but it'd be nice if we could have access to safe methods that don't panic if the key is missing.

[chyh1990]

It won't panic, it is one of the feature of yaml-rust. It just return a special Yaml::BadValue.

Example for readme:

// Chained key/array access is checked and won't panic,
// return BadValue if they are not exist.
assert!(doc["INVALID_KEY"][100].is_badvalue());

[blaenk]

Ah yeah, I forgot about that :)