title: Dash dropdown that allows multiple selection but is not clearable
body: <p>Is there a way to create a Dropdown menu that allows multi-selection, but does not allow the user to clear the selected options (i.e. have nothing selected)?</p>
  
  <p>As far as I can see, the multi flag overrides the clearable flag, i.e. the following code allows multi-selection and is clearable.</p>
  
  <pre><code>import dash_core_components as dcc
  
  dcc.Dropdown(
      id='column-plot-list',
      options=[{'label': i, 'value': i} for i in ['opt1', 'opt2']],
      value='opt1',
      multi=True,
      clearable=False
  )
  </code></pre>