Backport features to master

[thomas-profitt]

Bumps version to 1.6.0 and backports these features from recent work to master:

• Handling of more attachment types (this gets a lot of bots working)
• Handling of Slack's permissive ``` block syntax
• Converting <!channel> to @room (though the bridge doesn't create rooms in which ordinary users can @room, so this doesn't fully work)