support for varibales

# example:

### gruntfile.coffee

```coffeescript
module.exports = (grunt)->

  grunt.initConfig

    slim:

      dist:
        options:
          variables: 
            ext: ".min"

        files: "index.html" : "index.slim"
            
      dev:
        options:
          pretty: true
          variables:
            ext: ""

        files: "index.html" : "index.slim"

  grunt.loadNpmTasks 'grunt-slim'
```

### index.slim

```slim
link href="/assets/foundation#{ext}.css"
script src="/assets/angular#{ext}.js"
```