Automatically include <iostream> in godbolt link

[chiphogg]

This eliminates the main annoyance in my personal use of the godbolt
link, and I suspect many others' too.

Fixes #290.

[geoffviola]

I think it makes sense to optimize your experience. I typically use cout as well.

There may be some std::fputs users that prefer a smaller binary size. If there are issues, we can revisit that later.

[chiphogg]

Yep, I don't think it matters too much; all we need is a reasonable default.