Specify what whitespace is.

I had a hard time to get the desired result out of my template into a form which fits our codingstyle. I use jinja to generate C code. But in C empty lines should be preserved as much as possible to increase readebility. And while I'm not only generate C code but C header files which will be installed, these empty lines are of high value for our project. Unfortunately, the \s in pythons re engine also matches \n, thus any jinja token also eats up its leading empty lines.

This pull request includes my current solution to this problem. It lets the user specify what whitespace is. In my project I just use [ \t].