Added spec for :vk subscript adverbial
Added spec for :vk subscript adverbial

I assume we will need a .vk method later as well.  Will spec this later unless
someone beats me to it / tells me not to do it.

tokuhirom/specs,prammer/specs,perl6/specs,prammer/specs,tokuhirom/specs,tokuhirom/specs,prammer/specs,perl6/specs