Add customisable modalTransitionStyle

Adds the ability to easily customise the modalTransitionStyle exactly like the current modalPresentationStyle tweaks made 6 months ago... https://github.com/ShareKit/ShareKit/pull/482

Working on a ShareKit integration for Buffer that uses a different transition style and wasn't able to find anyway of doing it easily like the below code in my configurator.

```
- (NSString *)modalTransitionStyleForController:(id)controller
{
    if ([controller isKindOfClass:NSClassFromString(@"SHKBuffer")]) {
        return @"UIModalTransitionStyleCrossDissolve";
    }
    return [self modalPresentationStyle];
}

- (NSString *)modalTransitionStyle
{
    return @"UIModalTransitionStyleCoverVertical";
}
```