Added pause() and resume() methods

Added two new methods which allow you to pause() and resume() annyang.

These will just cause annyang to stop responding to speech commands, without actually stopping or restarting the browser's SpeechRecognition engine - which is great because you can now instantly pause-resume-pause-resume at any moment without requesting permissions again (even on HTTP).

This contribution comes from @niki4810 who also provided an interesting scenario for when and how to use this in #98

This release also catches exceptions thrown by the SpeechRecognition engine, if you try to start it when it is already started. The error message is logged to the console (if debug mode is on)