Support for Compass' compile -I IMPORT_PATH option

Adds support for the `compass compile -I IMPORT_PATH` option which allows specifying an additional import path folder which contents will be made available to the Compass project using the `@import` function in our Sass stylesheets.

To set:

```js
gulp.src('./src/*.scss')
  .pipe(compass({
    import_path: 'bower_components'
  }));
```

This would append the `-I bower_components` option to the `compass compile` command.