Setting/filtering output

These are feature requests that I would like to see (or know about if they already exist). If I get the time, I may work on them myself and post or PR.

Both features could be implemented by setting up a filter on all return statements.

I would like to truncate some of my error messages (ReferenceError: foo is not defined could be under 5 lines, for me, for example). I would also like to define a prompt for returned statements. It could look something like:

my_prompt> 2 + 2
=> 4

where '=>' is defined by the user.