The audiornnoise element can be used as playbin audio-filter for instance, to
suppress noise in the audio stream.

This plugin relies on the nnnoiseless crate which implements the noise removal
algorithm of the Xiph RNNoise library, in Rust. Some demos and documentation of
the RNNoise C library can be found on:

https://people.xiph.org/~jm/demo/rnnoise/

Fixes #114