Column ordering in Bootstrap 4 with push/pull and col-md-12

UPDATE (FEB 2018) - v4+

Now that bootstrap has been released, you can achieved that using order utility classes as you were able to do it in beta version (see old update below), with the difference that they've added these 3 new classes:

.order-first {
  -webkit-box-ordinal-group: 0;
  -ms-flex-order: -1;
  order: -1;
}

.order-last {
  -webkit-box-ordinal-group: 14;
  -ms-flex-order: 13;
  order: 13;
}

.order-0 {
  -webkit-box-ordinal-group: 1;
  -ms-flex-order: 0;
  order: 0;
}

Snippet

.p-2 {
  background: red;
  border: white 5px solid
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet" />
<div class="d-flex flex-column">
  <div class="p-2">1</div>
  <div class="p-2 order-first order-lg-2">2</div>
</div>

OLD UPDATE (OCT 2017) - v4.0.0 beta

With the release of beta version you can do this using flexbox utilities from bootstrap, such as flex-order

(see answer from @ZimSystem - to see solution with alpha version)

.p-2 {
  background: red;
  border: white 5px solid
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet" />
<div class="d-flex flex-column">
  <div class="p-2">1</div>
  <div class="p-2 order-1 order-lg-2">2</div>
</div>

Update 2021 - Bootstrap 5 beta

The order-* classed work the same way...

<div class="row">
        <div class="col-md-12">
            Col 1
        </div>
        <div class="col-md-12 order-first order-md-0">
            Col 2
        </div>
    </div>

Demo Bootstrap 5

Update 2019 - Bootstrap 4.3+

Now full width, 12 unit col-*-12 columns can be reversed using flexbox ordering.

In older Bootstrap 4 alpha and beta versions the ordering utils were flex-*...

<div class="row">
    <div class="col-md-12">
        Col 1
    </div>
    <div class="col-md-12 flex-first flex-md-unordered">
        Col 2
    </div>
</div>

Demo Bootstrap 4 Alpha

As of Bootstrap 4.0.0 the ordering utils are order-*...

<div class="row">
    <div class="col-md-12">
        Col 1
    </div>
    <div class="col-md-12 order-first order-md-2">
        Col 2
    </div>
</div>

Demo Bootstrap 4.1.0

See the docs at https://getbootstrap.com/docs/4.0/layout/grid/#order-classes