Add diff functions

[mhkeller]

Sometimes when I'm doing some data cleaning, I want to know the diff between my current attempt and a previous one. It would be interesting if there is an out function that would diff the two results.

For example, let's say you have a script that outputs this file

key,value
hello,1
hi,2

and then you make some changes and now it outputs this data

key,value
hello,1
hi,2
hey,3

This function would write out the diff of these two