playground: print(ln) / stdout / stderr output

[hajimehoshi]

Now the result of println is shown at the console, but there are mixed messages from the playground itself and from println. Could we have a separate space to show println messages?

Also, it'd be useful to have space showing stdout / stderr messages, but I don't know we really need this.

[dave]

This is fixed now.

[hajimehoshi]

Thank you!