Oja tidy up

* Do not mutate `Node` object when traversing with `Node.getAncestor`, re. https://github.com/guardian/scribe/pull/23#discussion_r9232045.
* Fix broken tests for <delete>/SPAN issue
* Add API for unwrapping elements
* Fix mutation in `Node.nextAll`
* Tidy up tests so patches are separate from features
* Fix bug with P mode when LI elements contain inline elements (with tests)
* Add contributing guidelines to enforce some consistency
* And more…