Instead of simple api object allow function that returns an object given options from the other side

To allow returning different api by version, allow function

``` js
iframeApi(myApi).then(...);
function myApi(clientOptions) {
switch (clientOptions.version) {
case '0.1.0':
return { foo: ...}
break;
case '0.2.0':
return { bar: ...}
break;
}
}
```