Show stdout and stderr from exec output

[rschroll]

In exec mode, stdout and stderr go to the JS console. This is okay for debugging, but it's not useful for the average user. At a bare minimum, it'd be nice to have print work.

Note that, in eval mode, these outputs can be sent to the (LiaScript-provided) console for a much better experience. I don't think this is available in the exec mode.

[rschroll]

Looking into this, I found that the obvious solution, calling send.lia on each output, doesn't work. Each call erases any previous output. This is probably not what's desired.

I can think of three ways to attack this, and they're are probably others. I'll list them here to start a discussion:

1. Use something other than send.lia. It looks like send.log, from the interactive code-blocks, isn't available in this environment. But I haven't found a full API for the send object, so I'm not sure what other options may exist.
2. Record all output and call send.lia with the full recording each time. This wouldn't be too difficult, but it could result in send.lia being called many times. Each time it (presumably) deletes the previous output and inserts new output. As the recorded output gets long, this could result in a lot of work for even minor outputs. I suspect that it wouldn't be a problem for sensible amounts of output.
3. Create a custom HTML element and insert the output into that. This means each output only needs a small modification, even if many outputs have been called. But it also means reimplementing the console object from the interactive blocks, which I suspect is non-trivial.