File::json() method to get json decoded data from a file.

[AustinW]

This adds a convenient File::json() method to retrieve json encoded data from a file.

Example of current flow:

$contents = File::get('sample.json');
$data = json_decode($contents, true);

New flow using File::json:

$data = File::json('sample.json');

This additional method will not affect current users and eases the development flow when accessing json files. Tests are included.