Improved position addon

I thought of some ways to improve the position addon. The code I wrote into _position.scss should allow the users to do the following:
- Define coordinates as *either* a coordinate list (as was already the case) *or* named variables -- `$t, $r, $b, $l` (you may want to write out those words, depending on your style). I've found this to be useful since I usually only want to declare a couple of coordinates, don't need to declare all four.
- Currently, a unitless zero is ignored -- but I find unitless zeroes handy, so I let them pass.
- Instead, `auto` is ignored -- and a shortcut `a` (which I thought would be useful when plugging in coordinates -- e.g. `5px a 15px a` -- if you don't want to use named variables). My reasoning was that `auto` is the default, so we don't need to create that rule. (But what if the value is already defined and I want to *change* it to `auto`? I think this addon is most useful for an initial declaration, combining position and coordinates. If I just want to change coordinates on an existing position, I will probably change the specific coordinate rule, right?)

Instead of keeping this to myself, I thought I'd hand it back to you. What do you think?