Provide a `load_file` method like YAML's

[keithrbennett]

One very common use case is to need to parse the contents of a JSON file.

As far as I know, this can only be accomplished in 2 steps, e.g. JSON.parse(File.read('something.json')).

YAML provides a convenient load_file method that combines these two. Obviously, it is trivially simple to implement.

Could we get a similar method (preferably with the same name) in this JSON library?

[hsbt]

#387 was merged now.