Add support for instance args.
Add support for instance args.

bigzachattack/dockerspawner,jlamcanopy/dockerspawner,bigzachattack/dockerspawner,ibab/dockerspawner,betatim/dockerspawner,jupyter/dockerspawner,Xarthisius/dockerspawner,Fokko/dockerspawner,minrk/dockerspawner,jlamcanopy/dockerspawner,jhamrick/dockerspawner,ibab/dockerspawner,anaderi/dockerspawner,jupyter/dockerspawner,minrk/dockerspawner,Xarthisius/dockerspawner,jhamrick/dockerspawner,anaderi/dockerspawner,quantopian/dockerspawner,betatim/dockerspawner,Fokko/dockerspawner,quantopian/dockerspawner