Allow overriding colors when importing SCSS file

### 1. Add `!default`
`!default` allows overriding variables that are defined before @import

In my project, I'm importing the SCSS file into another file, and I'd like to define custom variables there. By adding `!default`, if the variables are already defined, the previously defined variables are used.

Here's the code from my project:

```
$table-row-background: #ffffcc;

@import "../../datatables-src/css/jquery.dataTables.scss";
```

------

### 2. Customizable shading color

If I wanted to shade using blue, I could use:

```
$table-row-background: #eeffff;
$table-shade: #0000ff;
@import "../../datatables-src/css/jquery.dataTables.scss";
```
Merge pull request #21 from zackkatz/patch-1

Allow overriding colors when importing SCSS file