Couchdb 2483 add cors to fauxton

This adds CORS into fauxton in the config section, allowing users to enable and disable CORS.
Could you move the logic of the if statement into the serialise function and then just have a resulting boolean here. So in serialze you would have:

    var isCorsEnabled = false;
    if (!_.isUndefined(enable_cors) && enable_cors =="true") {
     isCorsEnabled = true;
    }
    return {
     isCorsEnabled: isCorsEnabled
    };