Range and point selections

Reflecting on the selector format used by the Annotator project, also in
use in Hypothesis, and taking inspiration from XPointer, I wonder whether
the model would benefit from selector types that address ranges and points.

The text position selector has start and end numbers. Collapsed to the same
value, these could specify a point.

However, the selector could also be generalized to a range selector that
has a start and end selector, each just a text point.

The benefit of this would be that other types of ranges of content could be
described by existing or new point selectors, without the need to redefine
a range for that content type.

Perhaps  one could use the existing fragment selector, referencing XPointer
with the conformsTo property, to make range descriptions for HTML, but
maybe there's value in incorporating a similar form into the model directly.

A number of the DPUB use cases on the wiki seem to involve ranges.