title: multiple show_on restrictions for CMB2
body: <p>I'm just getting going with the <a href="https://wordpress.org/plugins/cmb2/" rel="nofollow" title="CMB2 plugin">CMB2 plugin</a>. As far as I can tell, there's no way of applying multiple restrictions to a box. There's limiting by post type, and the two built-in <code>show_on</code> conditions (as per <a href="https://github.com/WebDevStudios/CMB2/wiki/Display-Options" rel="nofollow" title="Display Options">Display Options</a>, and you can add <a href="https://github.com/WebDevStudios/CMB2/wiki/Adding-your-own-show_on-filters" rel="nofollow" title="custom `show_on` filters">custom <code>show_on</code> conditions</a>. But when it comes to <code>show_on</code>, you can only pass one array with a key/value pair. I've tried passing an array of arrays and it doesn't seem to work, e.g.:</p>
  
  <pre><code>$cmb = new_cmb2_box( array(
      'id'                =&gt; 'slideshow_content_box',
      'title'             =&gt; __( 'Slideshow content' ),
      'object_types'      =&gt; array( 'page' ),
      'show_on'   =&gt; array(
          array(
              'key'       =&gt; 'id',
              'value'     =&gt; array( 30 )
          ),
          array(
              'key'       =&gt; 'page-template',
              'value'     =&gt; 'page_template.php'
          ),
      ),
      'context'           =&gt; 'normal',
      'priority'          =&gt; 'high',
      'show_names'        =&gt; true,
      'closed'            =&gt; false,
  ));
  </code></pre>
  
  <p>I've tried hacking this with the <code>cmb2_show_on</code> filter, but custom arguments seem to get stripped out.</p>
  
  <p>Is there a way of combining multiple <code>show_on</code> restrictions?</p>