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.
• EXPERIMENTAL: We now can 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')
• Windows: fix minor bug on Windows chat exports
• Fix badge in README
• Minor fixes to docs

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.38931% with 27 lines in your changes missing coverage. Please review.

Project coverage is 94.40%. Comparing base (5336d58) to head (680e980). Report is 1 commits behind head on develop.

Files	Patch %	Lines
whatstk/_chat.py	67.64%	6 Missing and 5 partials :warning:
whatstk/whatsapp/objects.py	85.36%	3 Missing and 3 partials :warning:
whatstk/whatsapp/parser.py	85.36%	4 Missing and 2 partials :warning:
whatstk/graph/base.py	66.66%	1 Missing and 1 partial :warning:
whatstk/whatsapp/generation.py	60.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #146 +/- ## =========================================== - Coverage 96.51% 94.40% -2.11% =========================================== Files 22 22 Lines 660 751 +91 Branches 107 137 +30 =========================================== + Hits 637 709 +72 - Misses 17 25 +8 - Partials 6 17 +11 ```

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[lucasrodes]

TODO:

• improve coverage of tests
• "name" of the chat is only exported as a user on iOS. On Android, it seems that the field 'username' is missing.