Add patch format.

[hoverlover]

This commit adds a patch format similar to diff:

<?xml version="1.0" encoding="UTF-8"?>
<foo>
-  <bar>one</bar>
+  <bar>two</bar>
</foo>

Just need to set Differ.format = :patch.

[echarp]

This looks just great, although without the console colors, it would be even better ;)

[hoverlover]

I actually like the console colors, and think they are helpful. With a simple example like above, it doesn't really matter. But when you have a long many lines of text and they are littered with changes, the colors really help make the additions and deletions stand out.