feat: allow typed interval sub-classes with OverlapDetector

[nh13]

Alternative to #34

Why I like this implementation:

1. One can subclass Interval and use it add them with OverlapDetector, such that the methods that return intervals are correctly typed (subclass)
2. Adds a generic class ItemizedInterval that allows a developer to create intervals that have an attribute of a specific type T. Using this class, a developer doesn't need to create a subclass and can "hang" any object type on an ItemizedInterval.

Why I don't like this implementation:

1. Breaking change: now we need to always specify the interval type when constructing an overlap detector

[nh13]

Closed as #34 is merged.