Explicit opt-in to autoinstantiation (included in the default document.html)
means Squire may be loaded in sites that themselves are loaded in iframes,
without unexpected consequences.
Resolves#104.
* If you load the squire.js script into a top-level page rather than an iframe,
it will add a Squire constructor to the global scope.
* The Squire constructor can be used to instantiate multiple instances on the
same page without having to load/parse/execute the full code every time.
* For each instance, create a new iframe, then call `new Squire( document )`,
with the document node for each iframe.
