[touch-events] TouchInit should perhaps include pageX and pageY (#28)

In #10 I added a constructor for Touch.

@foolip [says](https://codereview.chromium.org/1272603003/#msg8):
> most event constructors just allow all the values
> to be set to anything at all, even if that's something that couldn't be produced
> by a real event.

I was thinking we were following the pattern from MouseEvent here (and keeping with the pattern of document.createTouch).  

[UIEvents says](https://w3c.github.io/uievents/#interface-MouseEvent):
> Note: When initializing MouseEvent objects using initMouseEvent, implementations can use the client coordinates clientX and clientY for calculation of other coordinates (such as target coordinates exposed by DOM Level 0 implementations or other proprietary attributes, e.g., pageX).

But I guess that's a little different as the UIEvents spec doesn't specify the other attributes.  Should we just let script supply `pageX` and `pageY` too?  I don't see any problem with that.

@mbrubeck any opinion?

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https://github.com/w3c/touch-events/issues/28