Mixin defaults ...

### Changes:

1. SCSS test file.
2. Added `.sass-cache*` to `.gitignore`.
3. Added mixin default args.
4. Cleaned up comments (wrapping to 72 characters).
5. Added property descriptions.
6. Added link to W3.org docs.
7. Slighty modified logic for a few `@if ...` directives.

Initial discussion: mastastealth/sass-flex-mixin#5