Make possible to choose the extension strategy

Add a option to choose the extension strategy when using SCSS format.

The placeholder selector strategy doesn't work very well in non-modern browsers (aka IE8) since it may create huge CSS selectors chain and the browser will not parse they.

So, for this case is better to use the mixin strategy.

## USAGE

```bash
spriteful images/icons -f scss --mixin
```

Will generate a file like:

```scss
@mixin icons-sprite {
  background-image: image-url('sprites/icons.png');
  background-repeat: no-repeat;
}

%icons-sprite-new {
  @include my-sprite;
  background-position: 0px 0px;
}
```

The final user usage continue the same:

```scss
// in your SCSS code...
.button {
  @extend %icons-sprite-new;
}
```