Make specifying flags easier

[lazarljubenovic]

The fixed-grid mixin currently uses a few "flags". These are actually arguments which are set to false by default, and the consumer can set them to true individually easily because Sass allows specifying arguments out of order when the argument name is given.

  @include fixed-grid(
    3,
    $distribute-dangling: true,
    $dangling-at-beginning: true,
  );

However, the number of flags would eventually grow, and this can become cumbersome. It can be solved much easier:

@include fixed-grid(3, $flags: distribute-dangling dangling-at-beginning);

Just for convenience (and some Sassy source code fun), it wouldn't be hard to allow an alternative syntax with spaces (but using commas to separate flags):

@include fixed-grid(3, $flags: distribute dangling, dangling at beginning);

We could still support the current syntax in parallel or just drop it completely.