Ports from aria-toolkit and enhancements

Added audit for multiple explicit owners.
Added audit for aria-owns DOM descendant.

Enhanced (and renamed) "non-existent-labelledby" audit to cover all id reference types: 
 * aria-activedescendant
 * aria-controls
 * aria-describedby
 * aria-flowto
 * aria-labelledby
 * aria-owns

Added a few utility methods to support some of the new functionality.
Added IE11 support.
Added unit tests for RequiredAriaAttributeMissing audit.
Unit tests for all changes.
I agree that would be a good thing to do!

Although I tend to find that for things like this it becomes automatic to just type the "right" thing after a while. The trickier/more annoying things are things like wrapping and lining up arguments correctly; I've been spoiled by how good clang-format is at those things and I haven't found a comparable tool for JS. How good is fixjsstyle?