Add dirname of current src file to rewriter function arguments.

[evanminto]

I wanted to be able to add URL rewriting logic based on the file being processed, so that the task can CND-ify paths relative to the src file rather than relative to the base URL (as is the case for the case where the base option is used). To do this, I wrapped the rewriter function inside the files loop to create a rewriter function that includes the result of calling path.dirname() on the current src file. Since it's the second argument and the first one (url) remains unchanged, this change is fully backwards compatible.

I left the sample image file empty, but that might be confusing. I could replace it with a real image if that's better.

Let me know if any of this doesn't make sense or could be improved.

[XhmikosR]

@evanminto: can you rebase against master? Or enable edit by maintainers in this PR?

[evanminto]

@XhmikosR Oh, my bad! I checked the "Allow edits from maintainers" button, hope that works.

[XhmikosR]

All right, rebased. @evanminto: please look into the failures.