Add option to select audio source

Pass --audio-source=mic to capture the microphone instead of the device
audio output.

app/scrcpy.1

@@ -55,6 +55,12 @@ Use a specific MediaCodec audio encoder (depending on the codec provided by \fB\
 
 The available encoders can be listed by \-\-list\-encoders.
 
+.TP
+.BI "\-\-audio\-source " source
+Select the audio source (output or mic).
+
+Default is output.
+
 .TP
 .BI "\-\-audio\-output\-buffer ms
 Configure the size of the SDL audio output buffer (in milliseconds).