record.start() returns child process object

[mreinstein]

this fixes #19

I need this ability to detect when an error occurs (for example, the microphone is unplugged.) Being able to attach to stderr enables determining if the process closed normally or errored.

[gillesdemey]

I would prefer emitting an error event when sox writes something to stderr instead of changing the public interface; this would otherwise require a major version bump.

see https://github.com/gillesdemey/node-record-lpcm16/issues/19#issuecomment-297541840