add support for SegmentTemplate padding format string and SegmentTimeline

[mjneil]

This adds support for the following SegmentTemplate features

• template string identifier replacement with padding format tag support
  • $$
  • $RepresentationID$
  • $Number%0{x}d$
  • $Bandwidth%0{x}d$
  • $Time%0{x}d$
• build segments from SegmentTimeline element

[forbesjo]

Could you add jsdocs for the new functions? We'll get to the other functions later