Add option to specify file encoding

Hi, this just jumped to me and hope it is useful.

Recently when trying to integrate [sass-mq](https://github.com/guardian/sass-mq) to my project, I received the following error:

    Running "sass:dev" (sass) task
    Syntax error: Invalid US-ASCII character "\xE2"
            on line 155 of /Users/jeduan/Code/cadenadeoracion/bower_components/sass-mq/_mq.scss
            from line 7 of assets/sass/style.scss
      Use --trace for backtrace.

A quick googling reveals that the sass cli has the -E option to specify an encoding.

Using said option did make the problem go away, but it wasn't exposed in grunt-contrib-sass, so this pull request exposes the option.