Compute and report diffs when both .rs and .md are newer than tango.stamp

[pnkfelix]

I wrote the following in passing on the https://github.com/pnkfelix/tango-demo/ README:

I hope in the future to extend tango so that it will actually report some sort of diff between each of the generated products, so that the developer can then attempt to manually port the necessary change into whichever single target file makes sense and remove its partner from the filesystem.

It was a good idea when I wrote it, and it continues to be a good idea now.

(Especially if tango could additionally suggest to the user "just manually update one file and delete its partner to get around this error" when the diff appears small.)