Improve stylelint config to accept Sass at-rules

[kamilzyla]

Changes

Closes #285.

How to test

Initialize a fresh project and add the following code to app/styles/main.scss:

@mixin shadow {
  box-shadow:
    0 4px 8px 0 rgba(0, 0, 0, 0.2),
    0 6px 20px 0 rgba(0, 0, 0, 0.2);
}

.card {
  @include shadow;
}

Try to run rhino::lint_sass(). There should be no errors reported.

[jakubnowicki]

LGTM