insert back-reference as comment to the .R source files used in each generated file

Proof-of-concept implementation, uses a new tag type `"backref"` to record the value of `"srcref"` at the time the title is parsed.

I'm not sure if this collects all back references, but surely it should collect the file where the title is defined, and which is probably the one the user wants to look at first.

Still open:

- [x] Tests
- [x] Usage tests
- [x] Prepend file names with `R/`
- [x] NEWS bullet

Fixes #290.
It's a "special" tag, different from others, as it will be rendered as a comment. I'll change it anyway.