Add equal-width columns for flexbox grid

Created by: bassjobsen

see: https://github.com/twbs/bootstrap/issues/17131

suggested documentation:

### Example: Auto-width columns

Use the `col-{breakpoint}-auto` classes to create rows with 5, 7, 8, and 10 columns. Each column gets the same width.

<div class="bd-example-row">
{% example html %}
<div class="row">
  <div class="col-md-auto">1</div>
  <div class="col-md-auto">2</div>
  <div class="col-md-auto">3</div>
  <div class="col-md-auto">4</div>
  <div class="col-md-auto">5</div>   
</div>
{% endexample %}
</div>