add Renderer
Adds a renderer to this ReaderView and optionally a mediaQueryRule (CSS media query rule).
When responsiveRendererSelectionEnabled is true, the mediaQueryRule is used to evaluate if the renderer can be used or not. The rule is evaluated each time refresh() is called, but may also be evaluated on other occasions. Omitting the mediaQueryRule allows the renderer to be used anytime.
If multiple renderers are added and more than one rule evaluates to true, the renderer that can use as much as possible of the view element area is chosen. If there are several renderers that can use an equal amount of the rendering area, the first one added will be chosen.