Improve Iterator metadata collection

[nathanbabcock]

Closes #10

• Rewrite metadata collection to avoid an inner loop inside the constructor
  • On certain errors execution could block inside this inner loop, never yielding items to the Iterator
• Collect metadata into a single struct
• Add collect_metadata method to advance iterator past metadata section and return the finalized metadata
• Recognize audio streams and other stream types, though no parsing of their contents yet