Expose an oncePageLoaded function

[theninj4]

Expose a function that takes a callback and invokes it once and once only. Resolves #16

• If the page has already loaded, the callback should be invoked immediately
• If the page is still loading, as soon as it has finished both loading and processing any clientside JavaScript, it should invoke the callback
• If the page has failed to do anything 6 seconds after the initial page load and the callback has yet to be invoked, it will be invoked.

[elliotstokes]

👍