Add restrictedGlobals option

Keep from undesirable global variable assignment by configuration.

For example, underscore.string redefines "window._" if it is undefined,
to assign "window._.str".

    if (typeof root._ !== "undefined") {
        ...
    } else {
        root._ = { str : ... };
    }

Inference engine thinks "window._" assigned twice in this case.
For workaround, manually restricts global assignment to specific file
by below config.

    "restrictedGlobals" : {
        "_" : "lib/underscore.js"
    }

(Also accepts list of filenames.)