title: OR filter on MultipleChoices with django-filter
body: <p>I have a form field with choices like [1, 2, 3, 4+], that 4+ means <code>greater than equal</code> and multiple choices can be selected. I want to do the filter using <code>django-filter</code>.</p>
  
  <p>I could do the filter for [1, 2, 3], but I don't know how to <code>or</code> it with <code>gte=4</code>. </p>
  
  <p>both the following work for filtering [1,2,3]:</p>
  
  <pre><code>class NumberInFilter(django_filters.BaseInFilter, django_filters.NumberFilter):
      pass
  
  class PlanFilter(FilterSet):
      obj = django_filters.NumberInFilter(name='object', lookup_expr='in')
      class Meta:
          model = Plan 
          fields= ['object',]
  </code></pre>
  
  <p>or</p>
  
  <pre><code>choices= ( 
           (1,1),
           (2,2),
           (3,3),
           )
  
  class PlanFilter(FilterSet):
      obj = django_filters.MultipleChoiceFilter(name='object', choices=choices) 
      ...
  </code></pre>
  
  <p>So how can I filter the multiple choices with a <code>gte=4</code> field?</p>