accept existing iframe by id

At AJ English we usually already have the iframe in the page, all we need is to augment it with pym.js. This change allows you to skip the container element and the iframe creation when the supplied `id` points to an iframe element. 

Example:

```html
<iframe id="f2" src="frame.html"></iframe>
<iframe id="f3" src="frame.html"></iframe>
```

```js
//override iframe's @src
var pymParent2 = new pym.Parent('f2', 'frame.html?foo=bar', {});

//use iframe's @src
var pymParent3 = new pym.Parent('f3', null, {});

```

Note that the framed content requires `pym.Child()`, this is not a solution for https://github.com/nprapps/pym.js/issues/32