title: Django crispy forms not adding id to field
body: <p>I am trying to add a class to a field in my form. When I check the rendered template, it is missing the class i have added.</p>
  
  <p>form:</p>
  
  <pre><code>from crispy_forms.helper import FormHelper, Layout
  from crispy_forms.layout import Field
  
  class OperationsCalendarForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
          self.helper = FormHelper()
          self.helper.layout = Layout(
              Field('start', css_class="datetimepicker")
          )
  
          super(OperationsCalendarForm, self).__init__(*args, **kwargs)
  
      class Meta:
          model = OperationsCalendar
          exclude = ('creation', 'last_modified')
  </code></pre>
  
  <p>form.html</p>
  
  <pre><code>{{ form|crispy }}
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>&lt;input class="dateinput form-control" id="id_start" name="start" type="text"&gt;
  </code></pre>