Make breakpoint separator configurable

[patrickcate]

Thanks for making this great plugin for include-media. One feature I would love is adding a Sass variable to making the breakpoint separator configurable instead of the hardcode @ symbol.

I know that matches the BEMIT naming convention, but I've run into problems were CSS class sanitizers of some CMS's (Drupal specifically) currently strip characters which need to be escaped to work as CSS selectors.

Making this configurable would get around this issue and just generally add some extra flexibility to the plugin. I'd be willing to submit a PR to add this feature if you're amenable to it.