More readable diffs via difflet

Instead of outputting the raw YAML blocks when inequality errors occur, I modified tap-min to use [difflet](https://github.com/substack/difflet) to print differences.

I made this because one of my projects compared complex JSON and it was too hard to see the differences without some help.

Here's what the **current** version looks like:
![](http://i.imgur.com/GHhs1Cg.png)

Here's the **proposed** version:
![](http://i.imgur.com/ozkUQT5.png)

I'm not sure if this strays too much from your original design goals too much, but if you like the changes I'd prefer to merge them in rather than perma-fork.

Thoughts?