Allow customization of evaluated block format

[guibor]

This is a feature request.

For instance,

# #+begin_example
# =>
... 
# #+end_example

which would allow outorg to export the results in html or other formats nicely.

(much more advanced: if it were possible to choose a special output format for different types (e.g if an object is a vega-lite object, then somehow allow the right evaluation so that outorg could export to an svg using for example emacs-vega-view).