0.2.0

• Switched to 2018 edition.
• Renamed Subtitle to Item.
• Changed type of subtitle's start_time and end_time to Time.
  (You still able to convert it to Duration.)
• Renamed parse function to from_str.
• Renamed read_from_file function to from_file.
• Added from_reader function.
• Removed Result alias.
• Error struct replaced by a bunch of different structs.