Absolute, relative, and fixed positioning mixin

These are some helpers for absolute, relative, or fixed positioning. Kinda based on https://gist.github.com/jpweeks/6733356.

```scss
@include position($pos, $coords);
@include absolute($coords);
@include relative($coords);
@include fixed($coords);
```

`fixed`, `absolute`, and `relative` are just shorter aliases to their `position(relative, $coords)` counterpart.

### Examples

```scss
@include absolute(10px);
// outputs
position: absolute;
top: 10px;
right: 10px;
bottom: 10px;
left: 10px;
```

The 1-4 values match the behavior of defining padding and margins in clockwise order.

```scss
@include position(absolute, 10px 20px 30px 40px);
// outputs
position: fixed;
top: 10px;
right: 20px;
bottom: 30px;
left: 40px;
```

If you leave off the fourth value, the left value uses the right value.

```scss
@include fixed(10px 20px 30px);
// outputs
position: fixed;
top: 10px;
right: 20px;
bottom: 30px;
left: 20px;
```

If you leave off the third value, the bottom value uses the top value.

```scss
@include relative(10px 20px);
// outputs
position: relative;
top: 10px;
right: 20px;
bottom: 10px;
left: 20px;
```