Add ability to use activity directly without showing in a share sheet

I had a use case where I was showing the open in icon inside my own chrome, and wanted to just directly trigger the `documentInteractionHandler`, without re-writing the iPad/iPhone detection and the other niceties of this library.

This pull adds 2 methods:

```objective-c
- (instancetype)initWithActivityItem:(id)item inViewController:(UIViewController *)viewController andRect:(CGRect)rect;
```
This method just is an initializer with a single item and vc, which allows us to use this activityItem outside of a standard `activityItem` context (it stores the activityItems internally, automatically calling `canPerform` and `prepareWith` methods).

and

```objective-c
- (void)showDocumentInteractionController;
```

Which will just present the interaction controller with the `superViewController`, `superView` and `CGRect` currently set on the activity.