Support unsolicitied direct file uploads

[Ashex]

Resolves #429

Incorporate logic from #369 for sending direct messages and adding some logic to allow passing a user id into send_file that will trigger opening a direct message channel. This PR also adds support for handling messages from bots and webhooks.

Example code:

    @respond_to(r"send me a file")
    def test_send_file(self, message):
        """give me a file: Just send a file for testing."""
        r = requests.get('https://gist.githubusercontent.com/Ashex/703ef3311bcb4fa50d1268303f5b08ab/raw/87159c75045d723908488c0e9ce99774858f7fb2/guide.md')
        self.send_file(message=message, content=r.text, filename='readme.md', text='Here is your file!', 
                       filetype='text/markdown', is_direct=True, channel=self.get_user_id(message.sender.handle))

    def get_user_id(self, user_id):
        people_cache = self.load('slack_people_cache', {})
        for k, c in people_cache.items():
            if user_id == c.handle:
                logging.info(f'Found {c.id}')
                return c.id

Very open to feedback on this implimentation.

CC: @pastorhudson