Use dotty context functions for cleaner command handlers

[XDracam]

Obviously, this feature requires dotty.

By using Context Functions we can get rid of the implicit msg => that litters many a bot.

This example works in a dotty project:

  type MsgHandler = Message ?=> Future[Unit]
  def onCmd(filter: CommandFilterMagnet)(action: MsgHandler) = onCommand(filter)(implicit msg => action)

  onCmd("foo") { reply("bar").void }