Allow control over output and environment for nodes
Allow control over output and environment for nodes

Each node that creates a subprocess now has control over the output
handles and environment variables for the subprocess.  This will
enable environment-based logging (as ffmpeg uses) or logging based on
a file handle.

This is a step toward logging support.

Issue #12

Change-Id: I717d411e4b708eeaf05af03e1a16602336c2d065

shaka-project/shaka-streamer,shaka-project/shaka-streamer