tinycap: support streaming captured PCM to stdout

It is sometimes useful for testing to be able to stream the
captured PCM to another tool instead of being forced to save
it to a WAV file.

Using '--' instead of the target filename will send the raw
PCM to stdout (without any header).

Change-Id: Ie576708d1b493844c897c66a3e55cad76166c331
Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>