feature request: ion-infinite-scroll please add immediate-check attribute

I have battled an issue where the `on-infinite` function would call too soon on my views. My hackish solution to this issue was to call scrollTop on the views `$ionicScrollDelegate` after my view got JSON from an `$http` call - this prevented the `on-infinite` function from being called right away.

Finally I decided to look at the source for the `ion-infinite-scroll` and see if I could add a feature similar to what the `ngInfiniteScroll` angular directive offers ([please see their infinite-scroll-immediate-check option here](http://binarymuse.github.io/ngInfiniteScroll/documentation.html)).

Basically I added an attribute to the `ion-infinite-scroll` called `immediate-check` that will determine if the on-infinite function should be called on start after the scrollView is rendered.  It won't affect anyone that is not using the `immediate-check` attribute and allows for some finer tuning on the infinite scroll (without my hackish solution lol).