Add flex-grid size modifier

Add `flex-grid-size` to modify the size of a flex-grid column. It
allows to define responsive behaviors in a semantic sass.

For exemple:
```
.layout {
  @include flex-grid-row;

  // Mobile: menu above the content
  &__col-menu { @include flex-grid-column(12); }
  &__col-content { @include flex-grid-column(12); }

  // Desktop: menu on the left of the content
  @include breakpoint(medium) {
    &__col-menu { @include flex-grid-size(4); }
    &__col-content { @include flex-grid-size(8); }
  }
}
```