Add docs for attachments with authors

Merge pull request nexylan#20 from rap2hpoutre/patch-1

Make it compatible with the new guzzle version

Merge pull request nexylan#15 from iwiznia/master

Add support for attachment's title and title link

Merge pull request nexylan#14 from Leenug/add_image_to_attachment

Add image_url to attachment

Fix indentation

Update README

Document new unfurling and markdown options

Tidy up config file

Update README