Added --domdiff option to pass in diff-marked DOM for selser testing

* To be used like this:

node parse --html2wt --selser --oldtextfile src.wt --domdiff
diff.html < modified.html

Change-Id: I9dcef701de4b7146d4e9c8ae62bdf5fda1a2bcf3
Merge "Added --domdiff option to pass in diff-marked DOM for selser testing"