Add Dual Pane Support

Dual Pane view is a popular diff format.  This change adds support for dual pane view via the 3rd argument to diff (a boolean dual pane flag).

A dual pane diff returns two key-pairs.  The 'before' key's value is the before input with deletions.  The 'after' key's value is the after input with insertions.

Sample usage:
diff = require('./htmldiff.js');

console.log(diff('<div class="wiki"><p>this is some of the text</p></div>', '<div class="wiki xhtml"><p>this some more of the text</p></div>'));
console.log(diff('<div class="wiki"><p>this is some of the text</p></div>', '<div class="wiki xhtml"><p>this some more of the text</p></div>', true));