Use "expand" explicit value for default flex-grid-row size

I often have in my semantic Scss many column definition aligned, and I
think its behaviours could be more understandable with an explicit
value (like `expand`) for columns which take the remaining place,
instead of `null`.
*`expand` would be always the default behaviour*

Something like this, similar to what `foundation-apps` purpose :
```scss
.my-row {
  .col-a { @include flex-grid-column(1); }
  .col-b { @include flex-grid-column(expand); }
  .col-c { @include flex-grid-column(shrink); }
}
```

Instead of
```scss
.my-row {
  .col-a { @include flex-grid-column(1); }
  .col-b { @include flex-grid-column; }
  .col-c { @include flex-grid-column(shrink); }
}
```