Support per-frame QP configuration by VideoEncoder  #270

Customize BRCs by applications, in addition to CBR/VBR provided by VideoEncoder.

Some applications prefer to implement its own BRC according to per-frame QP, to better response to network bandwidth estimation. One such per-frame QP BRC implement can be found:
https://github.com/intel/libmebo/blob/master/explainer.md

dictionary VideoEncoderEncodeOptions {
  boolean keyFrame = false;

  // proposal
  unsigned long QP;
};