Font-face mixin – choose which formats to use

I don't know if I'm missing something here, but does the font-face mixin assume you have `.eot`, `.woff`, `.ttf` and `.svg` files for your fonts? I would like to be able to use only `.eot` and `.woff` for example. Is this possible?
Space after `$font-url` to match your `@mixin font-face ( ... )`