node : add audio_ctx and audio buffer params

[pprobst]

This PR does two things regarding the addon.node example.

• It adds the audio_ctx param.
• It adds the option to pass directly an audio array with param pcmf32, instead of reading from a file as usual.

I added the last option mostly because I'm using it myself, and it might be useful for others. This way, we can pass a WAV (mono channel) audio array directly to addon.node, without the need to read from a file.