Selectable should not overwrite the global mixin namespace
Runner should include singleselect
SingleSelect should have a default selectedLength of 0 and an empty selected reference
Calling singleselect#selectItem() should call select() on that item
singleselect.selectItem() should reference that item as selected
singleselect#selectItem() should dereference old selected
if a selectable is already selected, deselect it