Add stylus support for sprites

Adds support for stylus output. Generates placeholders - pretty much like the sass generator.

Sample output:
```stylus
$icons36_styl
  background: url("../img/icons36_stylus.png") no-repeat

$sprite-test_01
  @extend $icons36_styl
  background-position: 0 0

$sprite-test_02
  @extend $icons36_styl
  background-position: 0 -32px

$sprite-test_03
  @extend $icons36_styl
  background-position: 0 -64px

$sprite-test_04
  @extend $icons36_styl
  background-position: 0 -96px
```

See: https://github.com/asciidisco/grunt-imagine/issues/50