Updated start(), extended convenience methods, updated example and documentation

Main purpose is to allow a user to arbitrarily `start(time)` the timer at any time and with more string formats. Added more convenience methods to convert milliseconds to a timecode String and to extend msToTime to accept additional formats. Updated HTML example and documentation to reflect new changes.

- start() now accepts an arbitrary start time in Unix time.
- pause() updated with new logic
- added msToTimecode() method to convert milliseconds to HH:MM:SS String
- modified msToTime() method to allow new argument formats like HH:MM:SS
- modified _startTimer() method to handle new start() argument
- updated README to explain new functionality
- updated example.html to show new functionality