Add option to preserve blank lines

[k4b7]

I'd like to use escodegen for formatting code in Khan Academy's live-editor. Unfortunately, the current behaviour removes blank lines which we would like maintain if possible. Pull request #208 adds this feature.

[k4b7]

I created this issue in the hope that the review-ninja would see it. Next time I'll create the issue before the pull request.

[Constellation]

Ah, I've now noticed that your PR is now updated. Great. I'll look into it after today's work is done :)

[k4b7]

I just realized that this issue was still open. #208 has been merged so this can be closed.