added native event access for splitPanes hooks

[silicakes]

Allowing access to the mouse / touch native events

Before this PR, we couldn't have a way of short circuiting events using stopPropagation since the API wouldn't expose the original event.

If you'd add a 'draggable' feature, the two will mix, resulting the following:

This PR fixes it by exposing the underlying browser event and allowing developers to stop its propagation, along with all the benefits of having native event access.

• The existing test suites (yarn test) all pass
• For any new features, documentation has been added
• For any documentation changes, the text has been proofread and is clear to both experienced users and beginners.
• Format your code with prettier (npm run prettier).

[coveralls]

Coverage increased (+0.07%) to 91.489% when pulling 4244b4f on silicakes:master into eb6b2bc on tomkp:master.