Release v0.2.0

- Added VP9 encoding and decoding support via new Vp9Encoder and Vp9Decoder classes,
  following the same API as their VP8 counterparts.
- VpxEncoderConfig and VpxDecoderConfig are now shared between VP8 and VP9 — no
  duplicate configuration classes.
- VPX_EFLAG_FORCE_KF constant is now accessible from both VP8 and VP9 encoders.
- CI pipeline now automatically creates a GitLab Release (with JAR asset and CHANGELOG
  link) when a vX.Y.Z tag is pushed.