Line numbers

[ghost]

It would be really helpful if when using the linear layout, each line of code had the number it corresponds to on the left as, right now, without it, it is hard to get back to the original line of code.

[jashkenas]

That would be a nice feature, if added unobtrusively: if line numbers were only visible when you hovered over the line, for example.

Feel free to send in a PR that implements it, and I'd be happy to merge.