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: Ifb40c8d360620fad5cafa09fb22b9fdd1e5f14c5
Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>