Google Code Issue 157: Add "escape invisible characters" option

> Reported by @fantasai, Jul 27, 2010 
> Having invisible characters in the source code can be confusing to someone who's trying to figure out what's going on. Adding an escape_invisible option would make those characters visible in the source code.

> The attached patch implements an escape_invisible option. The list of invisible characters is probably incomplete for this iteration, but you get the idea. It depends on the patch in  issue 156 .