IPv6 support

Scheme layer used: 73

It's here, it has come! The library now supports IPv6! Just pass use_ipv6=True when creating a TelegramClient. Note that I could not test this feature because my machine doesn't have IPv6 setup. If you know IPv6 works in your machine but the library doesn't, please refer to #425.

Additions

• IPv6 support.
• New method to extract the text surrounded by MessageEntity's, in the extensions.markdown module.

Enhancements

• Markdown parsing is Done Right.
• Reconnection on failed invoke. Should avoid "number of retries reached 0" (#270).
• Some missing autocast to Input* types.
• The library uses the NullHandler for logging as it should have always done.
• TcpClient.is_connected() is now more reliable.

Bug fixes

• Getting an entity using their phone wasn't actually working.
• Full entities aren't saved unless they have an access_hash, to avoid some None errors.
• .get_message_history was failing when retrieving items that had messages forwarded from a channel.