Expose the current source and destination paths to the replacement function

[tatablack]

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.

[yoniholmes]

Hey just wanted to say sorry for the delays - have been particularly busy. Still going to be a little while before I can review and merge, there's a queue of pull requests so want to get through these at the same time. Thanks, Jonathan

[tatablack]

No worries.. by the way, I just noticed this feature is also what #2 is about.
You might still want to implement it differently (passing a single object to the to function with the current parameters as its properties, as you suggested here), though my solution might also be good enough.