Allow user to specify media options on request.

[allen-edia]

Hi,

This is intended to address #30.

The application code can specify a 'get_media_options' function during construction, which is passed the default_media_options and returns an arbitrary options object.

In our case we could request the maximum possible resolution from Firefox using the following code, after which it is no longer capped at 640x.

            get_media_options: (default_media_options) => lodash.merge(
                {},
                default_media_options,
                {
                    // --- request maximum possible resolution:
                    // necessary for Firefox, at least.
                    video: {
                        width: 9999,
                        height: 9999,
                    }
                }
            )

[kirantpatil]

Yes, even in latest Firefox 58 on android, I am getting this issue, but not on chrome browser.