Make this library (SASS source) extensible.

[bunomonteiro]

Q: What this PR fixes?
A: Allows add filter to an existing CSSgram filter (only using SASS)

Q: Why?
A: CSS3 filter property does not allow inheritance, you cannot 'add' new filters keeping the existing filters

Before:

figure {
  @extend %aden;
  &:hover{
    filter: blur(2px); // overrides Aden filter, now has only blur filter
  }
}

Now:

figure {
  @extend %aden; // or @include aden;
  &:hover{
    @include aden(blur(2px) /*...*/); // Aden filter still works, but now blur filter was 'added'
  }
}

PS: Now comments are compatible with SassDoc.

😄

[una]

@bunomonteiro, this is a great point -- do you mind updating the docs as a part of this PR with the updated usage?

[bunomonteiro]

Of course! Why not? 😉

[una]

Sorry for the wait on this -- I've been thinking it through since this is a major addition! I do like it a lot and it's backwards compatible with an additional consumption model I definitely agree is important to have.

Thank you for your careful consideration and documentation! ✨ This is great!

[bunomonteiro]

uhuu!

No need to apologize, it did not take much. It took the time needed to do a good job. 😃