Allow parsing from `std::io::Read`

[konstin]

Currently, simply reading a file to a JsonValue involves quite some boilerplate:

let mut contents = String::new();
let mut file = File::open(some_path)?;
file.read_to_string(&mut contents)?;
let json = json::parse(&contents)?;

This could be hugely improved by allowing parsing directly from a std::io::Read:

let json = json::parse(File::open(some_path)?)?;

[imp]

It may be useful to provide an load() convenience wrapper specifically for such task as it may be in high demand.

[89z]

alternative:

https://docs.rs/serde_json/latest/serde_json/fn.from_reader.html