Add easy to use preprocessor support

[virtuald]

• Fixes Issues with preprocessor support disabled #60

@tomazsustar let me know if this sufficiently addresses your concerns. I also have a version of this that will work with the web example too.

[tomazsustar]

@virtuald I tested it and I like the approach. I had issues with _filter_self on windows, wich forced me to use retain_all_content=True.

In my example, the output of the preprocessor only contained a single file. The #line directive was with absolute path and the _filter_self was looking for a relative path with a few '../' prepended.

I think the #line directive should be parsed and converted to a relpath, so that we are allways comparing the same thing. Or both paths should be converted to an absolute path.

[virtuald]

I don't have a windows machine nearby at the moment, but if you can provide the arguments you're providing to the functions I can try to reproduce. Or if you have a fix, that's great too.

We probably should make a testcase for this and add it to the tests.