Receive: Config of decode sample rate/channels (#265)

* Receive: Config of decode sample rate/channels

This PR allows for dynamic configuration of the output sample rate and
channel count of received Opus audio. Users who rely on supported
formats should no longer need to manually resample & downmix audio
decoded from SSRCs in a call.

Opus exposes tuples of (Mono, Stereo) x (8, 12, 16, 24, 48)kHz.
Changing this at runtime (mid-call) may cause some audio glitches, as
decoder state must be reconstructed from scratch for all affected SSRCs.

* Fix doc typo, consistent naming with MixMode.

src/driver/mod.rs

@@ -28,7 +28,7 @@ use connection::error::{Error, Result};
 pub use crypto::CryptoMode;
 pub(crate) use crypto::CryptoState;
 #[cfg(feature = "receive")]
-pub use decode_mode::DecodeMode;
+pub use decode_mode::*;
 pub use mix_mode::MixMode;
 pub use scheduler::{
     Config as SchedulerConfig,