Add ability to update the column width/offset without outputting all of the base column styles

In response to #249.

This allows you to update the width/offset of an already declared column without having to repeatedly output the styles seen below:
```scss
.foo {
    ...
    clear: none;
    text-align: inherit;
    padding-left: 0;
    padding-right: 0;
    *zoom: 1;
}
.foo:before, .two:after {
    content: '';
    display: table;
}
.foo:after {
    clear: both;
}
```
It saves 11 lines of code when sass is set to output 'expanded' code. As you can see, this would be a huge saving for large applications that might have hundreds of column declarations, all being updated as they cross breakpoints. Even with minifcation this would save a lot of KB.

The only other thing I want to change is the :last-child output, as that is often duplicated too. But it's late and I'm tired so I'll try tomorrow.

Let me know what you guys think :smiley: