Support `every` keyword for second argument to loop statements

[dylanbeattie]

First noted in cwfitzgerald/rockstar-webpiler#9

You can do this:

hash at "hello" is "world"
for value and key of hash
shout key with ":" with value
yeah

for every value and key of hash
shout the key with ":" with the value
yeah

But you can't you can't put 'every' before the SECOND loop variable:

for value and every key of hash
shout the key with ":" with value
yeah

Probably makes sense to add this, since I don't think it'll break anything and a lyric like For love and every beat of your heart kinda works.