bat: Add 'readcapture' option to support analyzing external audio

bat: Add 'readcapture' option to support analyzing external audio

If audio data is captured on another device (ie we are NOT using
loopback mode), then allow alsabat to analyze that data, by passing
a filename reference on the command line.

Add the '--readcapture' option to the argument parser. When
this option is specified, avoid doing a local capture, and instead
just read the audio data from the indicated file, and analyze that.

Fixes: https://github.com/alsa-project/alsa-utils/pull/166
Signed-off-by: Tim Bird <tim.bird@sony.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>