Support --update like original sass command.

[zw963]

Hi, I use emacs, which have a scss-mode, When I change my scss file, and save it,
this mode will invoke command like following to generate css

 ╰─ $   scss --update app/javascript/application.scss
      write app/javascript/application.css
      write app/javascript/application.css.map

Or compile directory which scss exist

 ╰─ $   scss --update app/javascript

sassc is not support this args

[xzyfer]

This feature was added to Ruby Sass because of the slow compilation times.

Since SassC is significantly faster than Ruby Sass we don't plan to work on this feature but would happily accept a pull request.