Allow { target, anchor } initialisation option for el (gh #625)

You can now use this for el:

```js
var ractive, 
	target = document.createElement('div'), 
	anchor = document.createElement('div');

anchor.innerHTML = 'bar';
target.appendChild( anchor );
fixture.appendChild( target );

t.equal( fixture.innerHTML, '<div><div>bar</div></div>' );

ractive = new Ractive({
	el: { target: target, anchor: anchor },
	template: '<div>foo</div>'
});

t.equal( fixture.innerHTML, '<div><div>foo</div><div>bar</div></div>' );

```
Closes #625