Using Sass Placeholders for proper use of @extend

[digitalkaoz]

this PR exposes all grid related class (.row, .col, s1-12, m1-12, l1-12) as Sass Placeholders:

%row, %l17...

see this http://12devs.co.uk/articles/semantic-grids-with-sass-loops-and-silent-placeholders/ for a proper explanation

this avoids extremely large selector chains when using the materialize grid classes in your sass files:

//bad
.my-row {
  @extend .row;
  
  .my-col {
    @extend .col;

    &.my-s7 {
       @extend .s7;
    }
  }
}

//good 
.my-row {
  @extend %row;
  
  .my-col {
    @extend %col;

    &.my-s7 {
       @extend %s7;
    }
  }
}

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[DanielRuf]

There is an error. Please keep an eye on the tests.

[digitalkaoz]

@DanielRuf fixed it, creating PRs through the github is not always cool ;)