Add a :compact option to strip whitespace off terminal nodes in pretty.rb

[shreeve]

I didn't see a way to do this, so I am submitting a PR to add the ability to strip whitespace off terminal nodes when pretty printing.

For example, instead of this:

foo Hello
bar World
baz
  bat Wow!

generating this:

<foo>
  Hello
</foo>
<bar>
  World
</bar>
<baz>
  <bat>
    Wow!
  </bat>
</baz>

The compact: true option would generate:

<foo>Hello</foo>
<bar>World</bar>
<baz>
  <bat>Wow!</bat>
</baz>