can't set stickyHeaders` in Storyboard for IGListCollectionViewLayout

## New issue checklist
`stickyHeaders` is a readonly property and if we initialize it from XIB or storyboard. we have no chance to set it to YES or modify it.

```
- (instancetype)initWithCoder:(NSCoder *)aDecoder {
return [self initWithStickyHeaders:NO topContentInset:0 stretchToEdge:NO];
}
```
- [TRUE ] I have reviewed the [`README`](https://github.com/Instagram/IGListKit/blob/master/README.md) and [documentation](http://instagram.github.io/IGListKit)
- [ TRUE] I have searched [existing issues](https://github.com/Instagram/IGListKit/issues) and this is not a duplicate

### General information

- `IGListKit` version:
- iOS version(s):
- CocoaPods/Carthage version:
- Xcode version:
- Devices/Simulators affected:
- Reproducible in the demo project? (Yes/No):
- Related issues:

### Debug information

```bash
# Please include debug logs using the following lldb command:
po [IGListDebugger dump]
```