Add offset mixin

An attempt to fix #191.

This PR adds the `offset()` mixin which can be accessed via:

```scss
@include offset($ratio: 1/4, $col-or-span: column, $gutter: $jeet-gutter);
```

I've tested it on CodePen and it seems to work. **However** I'm worried that I've basically just duplicated a lot of code in order to make this happen. It seems to me that maybe Jeet could do with some refactoring.

Also, I'm pretty tired so it might be that my implementation is a bit inefficient. I'll probably wake up in the morning and see it more clearly :smile: