title: MixItUp with two select / dropdown fields
body: <p>I'm not advanced enough in JS to get this working fully. </p>
  
  <p>I have two select fields with options:</p>
  
  <pre><code>    &lt;select name="" id="filter-position"&gt;
        &lt;option value="all"&gt;Filter by position&lt;/option&gt;
        &lt;option value=".instructor"&gt;Instructor&lt;/option&gt;
        &lt;option value=".leader"&gt;Leader&lt;/option&gt;
        &lt;option value=".blah"&gt;Blah&lt;/option&gt;
        &lt;option value=".whatever"&gt;Whatever&lt;/option&gt;
      &lt;/select&gt;
  
      &lt;select name="" id="filter-location"&gt;
        &lt;option value="all"&gt;Filter by position&lt;/option&gt;
        &lt;option value=".portland"&gt;Portland&lt;/option&gt;
        &lt;option value=".missoula"&gt;Missoula&lt;/option&gt;
        &lt;option value=".chicago"&gt;Chicago&lt;/option&gt;
        &lt;option value=".newyork"&gt;New York&lt;/option&gt;
      &lt;/select&gt;
  </code></pre>
  
  <p>The container where the filtered items live looks a little like this:</p>
  
  <pre><code>    &lt;ul id="filter-container"&gt;
          &lt;li class="mix"&gt;...&lt;/li&gt;
          &lt;li class="mix"&gt;...&lt;/li&gt;
          &lt;li class="mix"&gt;...&lt;/li&gt;
          &lt;li class="mix"&gt;...&lt;/li&gt;
          &lt;li class="mix"&gt;...&lt;/li&gt;
          &lt;li class="mix"&gt;...&lt;/li&gt;
      &lt;/ul&gt;
  </code></pre>
  
  <p>I can get each one to filter properly but not together (i.e., AND). The JS code I'm using goes like this:</p>
  
  <pre><code>  $(function(){
      var $positionSelect = $('#filter-position'),
          $locationSelect = $('#filter-location'),
          $container = $('#filter-container');
  
      $container.mixItUp({});
  
      $positionSelect.on('change', function(){
        $container.mixItUp('filter', this.value);
      });
  
      $locationSelect.on('change', function(){
        $container.mixItUp('filter', this.value);
      });
    });
  </code></pre>
  
  <p>If I filter by one select, things filter properly. If I then filter by the other, it overrides the first filter. What I want is for them to both be used.</p>
  
  <p>I know there's a way to get this to work, but I'm not sure how to get that concatenated string to do the "and" logic as documented in the <a href="https://mixitup.kunkalabs.com/learn/tutorial/advanced-filtering/" rel="nofollow">Advanced Filtering</a> section of the docs using <code>&lt;select&gt;</code>. Help?</p>