libx264: provide option to choose preset; default to something other than ultrafast?

[rfht]

Currently, fauxstream has preset ultrafast hardcoded for libx264 encoder. This may not be the best choice for a default, as quality is low and the size of the data will be large. See the video referenced in #5 (comment) for discussion/explanation, including some comparison of different presets.

Ideally, we would want to add a flag to choose a preset, and have the most usefult preset as the default, which is probably not ultrafast. Maybe superfast...

[morgant]

@rfht Good point! I'll do some testing with the superfast preset on my low-end hardware and also think about adding a preset flag.