Add @content to the font-face mixin

- This allows additional properties to be include with the mixin and output
  within the `@font-face` declaration
- Closes #806

core/bourbon/addons/_font-face.scss

@@ -1,7 +1,8 @@
 @charset "UTF-8";
 
-/// Generates an @font-face declaration. Accepts arugments for weight, style,
-/// usage with the Rails Asset Pipeline and file formats.
+/// Generates an @font-face declaration. Accepts arguments for weight, style,
+/// usage with the Rails Asset Pipeline and file formats. Also possible to
+/// pass additional `@content` to the mixin, for instance `unicode-range`.
 ///
 /// @argument {string} $font-family
 ///
@@ -58,13 +59,13 @@
     font-family: $font-family;
     font-style: $style;
     font-weight: $weight;
-
     src: _font-source-declaration(
       $font-family,
       $file-path,
       $asset-pipeline,
       $file-formats,
       $font-url-prefix
     );
+    @content;
   }
 }