* Deprecated: $$mime$$, $$html$$, $$svg$$, $$png$$, $$jpeg$$, $$mimer$$,
  $$defaultMimer$$, $$async$$ and $$done$$.

* NEL sessions: Added global object $$ to interact with session (deprecated
  objects like $$html$$ have been replaced by functions like
  $$.html(htmlString)).

* Custom output: Added convention to customise the output of class instances.

* Asynchronous output: Added convention to choose output cell.

* Allow requests to complete without outputting a result.

* Allow only one output result per request.