Change RangeController selection to null and restrict selection to content

This changes the default value of .selection null, forcing users to set the initial .selection to a compatible type (Array, Set, ...). Specs have been expanded to cover selection as Array and selection as Set.

Additionally, .selection is now constrained to elements that are present in .content.

See also https://github.com/montagejs/native/pull/4 and https://github.com/montagejs/matte/pull/30