fix Sass compilation error when setting 'auto' value, typos, and !import...

...ant declaration option
What I mean is, should the $important param take a boolean value, rather than a string which must be `!important`. Boolean params are sometimes hard to understand in code, but here is makes sense to use a named param since now it will come after `$use-px-fallback`:

```scss
@include rem('font-size', 24px, $important: true);
```