Support url parameter to turn autoplay on/off

[henrikingo]

Currently the autoplay plugin looks for the data-autoplay attribute on the root element and steps to determine if autoplay should be on.

It would be useful to add a run-time configurability option into the URL, so that I can create links to a presentation that will then start, or not, to autoplay depending on the URL value.

Example: http://example.com/presentation.html?impress-autoplay=5#/step-1

(The above will not work for local files, but that's ok. I'd rather not append to the hash part, as it would introduce much new complexity.)

[henrikingo]

Moved to upstream: impress#720