Fix NPE reporting unmatched text or comment.

The `t/compare.js` function would dereference null when a comment or
text node was unmatched.

Closes #66.
Display documents when `t/compare.js` fails.

Dump the expected and actual documents to standard out when
`t/compare.js` fails.
Implement `elseif` directive.

Closes #70.
Merge branch `elseif` into `redux`.
Merge branch `redux` into `master`.