Add 'comment' support

Added

• NodeList<T>.Slice(int, int) method to support ranges in C#8 (#89).
• comment field on data elements (and in 2.03 schema) (#108).

Changed

• Renamed BattleScribeVersion static well-known values from V0_00 to V0x00 (#86).
• Renamed Resources to XmlResources (.Source library) (#86).
• Changed NodeList<T>.GetEnumerator() and ListNode<T>.GetEnumerator()
  return type to custom enumerator NodeList<T>.Enumerator that's optimized
  for performance (#89).
• Changed all parameter names across the board to be camelCased. Also changed
  parameter names of With methods to value to mirror setters (#90).
• All Node With methods for collection properties are now extension methods,
  with the exception the ones where parameter name is the same as the property's
  that's being modified (#90).
• Renamed a couple of Source Core/Node properties (#91):
  • Category: IsPrimary -> Primary
  • CategoryLink: IsPrimary -> Primary
  • EntryBase: IsHidden -> Hidden
  • Repeat:
    • Repeats -> RepeatCount
    • IsRoundUp -> RoundUp
  • SelectorBase: PercentValue -> IsValuePercentage
  • SelectionEntryBase: Import -> Exported