encoder: add tuning options API.

encoder: add tuning options API.

Add encoder "tune" option to override the default behaviour that is to
favor maximum decoder compatibility at the expense of lower compression
ratios.

Expected tuning options to be developed are:
- "high-compression": improve compression, target best-in-class decoders;
- "low-latency": tune for low-latency decoding;
- "low-power": tune for encoding in low power / resources conditions.