Add "desktop_notification" event (without contents), "member_joined_channel" and "goodbye"

[joranvar]

Example of the contents of the event:

Object
  (fromList
    [ ("is_shared"        , Bool False)
    , ("ts"               , String "1564031630.000800")
    , ("is_channel_invite", Bool False)
    , ("channel"          , String "A1B2C3D4F")
    , ( "launchUri"
      , String
        "slack://channel?id=A1B2C3D4F&message=1564031630000800&team=5G6H7I8J9"
      )
    , ("imageUri"   , Null)
    , ("msg"        , String "1564031630.000800")
    , ("content"    , String "Test")
    , ("subtitle"   , String "joranvar")
    , ("event_ts"   , String "1564031630.000300")
    , ("title"      , String "Workspace title")
    , ("type"       , String "desktop_notification")
    , ("ssbFilename", String "knock_brush.mp3")
    , ( "avatarImage"
      , String "https://avatars.slack-edge.com/2020-12-31/someImageFile.jpg"
      )
    ]
  )

[joranvar]

Just added this, so it doesn't bother me for every event :)

[mitchellwrosen]

Sorry, I just created a merge conflict for you. Can you rebase @joranvar?

[joranvar]

No problem!

[joranvar]

Done! Should I split this into three PRs?