Feature: Provide included files

Hi, this is my pull request as discussed in #279. **Please note that this pull request is not finished yet, see below.**

I was working on this feature the past days when I realized that the code would get messier if I'd just pass another argument to bindings.render() (which has already seven arguments). Additionally I found it hard to grasp with optional arguments. That's why I decided to refactor the binding so it accesses the options object via property names instead of array indices.

Furthermore I noticed that the newly introduced `renderFile` (#285) doesn't normalize the passed-in options. Since this doesn't seem to be intended to me I decided to normalize them too.

As you can see I just refactored the code without touching the tests. Now all tests are running again, so this is currently save to merge back into the master (to prevent merge conflicts). Just wanted to get early feedback if you're ok with my refactoring.

Could you also check the C-code because I'm not an experienced C-programmer and might have missed something.