add option for adding diff headers

[john-behm-bertelsmann]

It might be nice to be able to add diff headers with an option.
Same as the linux diff utility does.

--- a/internal/diff/unified.go
+++ b/internal/diff/unified.go

[znkr]

Indeed. I didn't implement that because these headers are very application specific and easy to add manually. But I agree that it fits.