Add audio recording mode

[danielpaulus]

I found out that it is possible to only activate the audio recording mode.
If the HPD1 commands are never sent, the devices starts streaming audio samples right away. The cool thing about that is, that the device does NOT modify the status bar (9:41am, no cell carrier symbol, battery always full and such).

This PR introduces the new audio command that let's you save audio streams to mp3, wav or ogg files.

[codecov]

Codecov Report

Merging #61 into master will decrease coverage by 1.28%.
The diff coverage is 15.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   56.92%   55.64%   -1.29%     
==========================================
  Files          35       36       +1     
  Lines        1674     1727      +53     
==========================================
+ Hits          953      961       +8     
- Misses        639      683      +44     
- Partials       82       83       +1     

Impacted Files	Coverage Δ
screencapture/gstadapter/gst_adapter.go	13.40% <0.00%> (ø)
...encapture/gstadapter/gst_pipeline_builder_linux.go	0.00% <0.00%> (ø)
screencapture/coremedia/avfilewriter.go	71.73% <20.00%> (-6.84%)	⬇️
screencapture/messageprocessor.go	54.67% <100.00%> (+1.85%)	⬆️

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