Use col-md- instead of col-sm- in page.html.twig

This is more of a suggestion / query actually.

The module currently sets the responsive display for the main section and side bars to col-sm-9 and col-sm-3.

<section class="col-sm-9">
<aside class="col-sm-3" role="complementary">

But if you look at the getbootstrap website, it uses col-md-9 and col-md-3 instead.

<div class="col-md-9" role="main">
<div class="col-md-3" role="complementary"> 

This setting seems to work better across devices.

Modifying page.tpl.php is always an option.
But col-md-9 and col-md-3 seem to be the bootstrap standard.

It would also be great to have the option of adding the hidden-* classes to the columns, to hide the sidebars altogether instead of stacking them vertically.

Would appreciate your thoughts on both issues.

Thanks for all the great work on the module!