Add support for phantom's #renderBase64.

The options for `#render_base64` and `#render` are the same. They are aliases
of `#save_screenshot` and `#save_screenshot_base64` respectively. This closes

Usage:

    image = @driver.render_base64
    some_io.write image

    image = @driver.render_base64 full: true
    some_io.write image