Just return a boolean for the TreeWalker filter fn. This diverges from the spec,
but since the goal of this implementation is not to fully implement the spec
and we're never going to use a native implementation, this doesn't matter and
the code is easier to read when the function is just returning a boolean like
any normal filter function.
* 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.
Firefox incorrectly goes back/forward in history instead of moving the cursor to
the beginning/end of the line when you press cmd-left/right on a mac. We now
override this to do the right thing.
