Add support for setting command-line options via a JSON config file

The file's called .mogenerator-args, format is a simple JSON array. A
typical example might be:

    [
        "--template-var", "arc=true",
        "--model", "MyApp.xcdatamodeld/MyApp.xcdatamodel",
        "--machine-dir", "Model/machine",
        "--human-dir", "Model"
    ]