Respect json struct tags

[lestrrat]

One of the reasons I use https://github.com/ghodss/yaml is because it supports the json struct tag -- so I don't have to have multiple definitions of the same fields:

// I DO NOT want this
type Foo struct {
    A `json:"foo" yaml:"foo"`
    B `json:"bar,omitempty", yaml:"bar,omitempty"`
    ...
}
// I WANT this to work with this YAML library
type Bar struct {
    A `json:"foo"` 
    B `json:"bar"`
}

I would really like this library to (1) by default look at the YAML tag, and (2) if not found, look at the JSON tag