sm: Implement global.$$mime$$ convention
sm: Implement global.$$mime$$ convention

* Implemented the global.$$mime$$ convention. If after running a
  Javascript cell, global.$$mime$$ is defined, then its value is used as
  the MIME output.

* Example of use:
      var svg = "<svg><rect width=80 height=80 /></svg>"
      global.$$mime$$ = {"text/html": svg};
  outputs a black square.

* Note that global.$$mime$$ is deleted after each run.

kebot/ijavascript,kebot/ijavascript,srounce/ijavascript,srounce/ijavascript