Add methods to further modify section providers in a section controller

Methods to move section providers:
```swift
func moveSectionProvider(_ sectionProvider: CollectionSectionProvider, afterSectionProvider: CollectionSectionProvider)
func moveSectionProvider(_ sectionProvider: CollectionSectionProvider, beforeSectionProvider: CollectionSectionProvider)
```

Methods to insert section providers at arbitrary locations
```swift
func insertSectionProvider(_ sectionProvider: CollectionSectionProvider, afterSectionProvider: CollectionSectionProvider)
func insertSectionProvider(_ sectionProvider: CollectionSectionProvider, beforeSectionProvider: CollectionSectionProvider)
```

____
Not providing methods with indices since only the Section Controller or classes that inherit from it should call these methods and should therefore have access to all section providers. If indices were added, that adds the possibility for a section provider to be "greedy" and move itself to the first position, for example:

```swift
/// Some section provider
sectionController.moveSectionProvider(self, toIndex: 0)
```

Therefore, no index methods will be added.

These new methods are planned for `v1.1`.