Slack-cleaner2

fixes 41 by adding the missing `requests` dependency

migrate to https://github.com/slackapi/python-slack-sdk (see #38)

* implements a proper pagination for listing messages, channels, replies, and files

* refactored and cleaned up the required scopes
* migrated documentation to custom slack app only
* add error messages with required scope hints
* make `as_user=True` default to avoid invalid argument error

* add a new `.files` field to `SlackMessage` containing all the files attached to the message
* add new options to `SlackMessage.delete` `files=True` will also delete attached files, `replies=True` does the same thing for the replies aka. thread.

Thus, deleting everything possible should be reduced to:

py
s = SlackCleaner2(TOKEN)
for msg in s.msgs():
msg.delete(files=True, replies=True)