Add support for multiple VAAPI devices

[Brainiarc7]

As requested by @joeyparrish , I'm opening this issue to highlight the case for supporting multiple devices for VAAPI hardware based acceleration. See #5 for the comments on the same.

At the moment, VAAPI-based encoding is supported, but this implementation assumes platform defaults which could fail in cases where multiple VAAPI-capable devices exist or where specific VAAPI drivers need to be set on a per-session basis.

[joeyparrish]

Thanks! We'll keep this in mind. In the mean time, we are always open to contributions!