Add colored output

[NathanReb]

Having the usual red/green for removal/addition diff colouring would make the api-diff tool output much more readable.

There's to way to go about this:

1. patch diffutils so it supports colors
2. drop the dependency on diffutils and re-implements the small bits of it we use

If there is still no plan to release diffutils, it's probably best to just go with 2 here since we use a very small subset of it that's made of its simplest parts.