Expose the current source and destination paths to the replacement function

In my use case, I was using the replacement function to manipulate the file text, and I needed to know *which* file I was editing (full path).

So I exposed the current file path and the current destination path in a way that makes them accessible to the replacement function without having to propagate them all the way down. Let me know if you think this is a suitable solution, or if you'd rather add an additional parameter and propagate it.

I added a note about it in the README.