Allow step-size in @for block

[thany]

It would be nice to be able to declare a step-size when looping through a number with a @for block. I'm thinking maybe something like this:

@for $var from 10 through 90 step by 5 {
  //awesomess
}

Would be an easier / more readable / more declarative way to write:

$var: 10
@while $var <= 90 {
  //awesomess
  $var: $var + 5;
}

[cimmanon]

Duplicate of #691

[nex3]

Specifically see this comment.