settings: introduce `maxAspectRatio` option.

Allows the aspect ratio of the embedded video to be limited. For example,
if using:

    $('.selector').fitVids({'maxAspectRatio': 0.5625});

Embedded videos encoded at aspect ratios "squarer" than 16x9 will be vertically
letterboxed, i.e.: displayed centred in the `.fluid-width-video-wrapper` with
black edges down the side.