enhance: read from zip, use system messages

[lucasrodes]

Major enhancements:

• Rename of core function df_from_txt_whatsapp → df_from_whatsapp (keep old name as alias)
• Ability to read from a .zip file, just how the chat is exported on an iOS device.
• We now consider system messages:
  • In df_from_whatsapp: Use message_type=True to label each row as 'system' or 'user` depending on whether the message is from the user or the system.
  • in WhatsAppChat.from_source: Use extra_metadata=True to create additional class attributes chat.df_system (table with all system messages) and chat.name (name of the chat)
  • NOTE 1: System messages are identified by their username (they use the name of the chat).
  • NOTE 2: extra_metadata only makes sense when working with group chats.
  • NOTE 3: Group chats are identified as "chats with more than 2 users (including 'system')