control = (selector) => {

        if (typeof selector !== 'string') return;

        // Get an up-to-date list of elements.
        let elements = [].slice.call(document.querySelectorAll(selector));

        // If the registry exists, update the elements.
        if (selectors.history.indexOf(selector) > -1) {
            selectors[selector].elements = elements;
        }

        // If it doesn't exist, create a new registry.
        else {
            selectors[selector] = Registry(elements, options);
            selectors.history.push(selector);
        }

        return selectors[selector];
    }
The main interface. Take a selector and retrieve
the associated registry or create a new one.