reduce with initial value

[dominictarr]

I was about to merge @kemitchell's pr #77
but it was bugging me that there where two completely separate code paths, and that it added trippled the number of lines.

This takes a different style, the main thing, is that if acc is not provided, it treats the first item as a special case, then falls back to the main way. I feel that this is more the pull-stream way

I also changed the way variable arguments where handled, there are lots of ways to do this... This is just the way that I always do it, which i learned from isaacs back in the day, eg

[kemitchell]

Should have mentioned this weekend's handiwork here: https://www.npmjs.com/package/pull-reduction

[kemitchell]

I ended up doing this: https://github.com/kemitchell/pull-reduction.js/blob/727f96ac0b95ae59f17c47988258560d36622682/index.js#L90

But please feel free to take whatever you can from this PR! It is a humble offering. No feelings will be trampled in any event.