title: Using extra_columns in django_tables2.tables.Table
body: <p>Trying to use extra_colums and get no error but the table does not show up. I've use the documentation from <a href="http://django-tables2.readthedocs.io/en/latest/pages/api-reference.html#table" rel="nofollow noreferrer">here</a>. I am trying to add a column which will have checkboxes into the table. I have already predefined the table and can exclude some fields but with using the documentation I cannot figure out how to add a new column. I must be missing something</p>
  
  <p>The implementation can be seen below. Would appreciate any assistance. Thanks</p>
  
  <p>IN TABLES.PY</p>
  
  <pre><code>import django_tables2 as tables
  from project.models import Project
  
  class ProjectTable(tables.Table):
     project_name = tables.TemplateColumn("""
          {%if record.department == "TRACER"%}
                  &lt;a href=" {% url 'project_detail' record.id %}"&gt;
                      {{ value }}
                  &lt;/a&gt;
          {%else%}
             {{value}}
          {%endif%}
  
          """, orderable=True, accessor='name', verbose_name="Project 
          name")
  
     project_type = tables.TemplateColumn("""
          {% if value == 'Yes' %}Special{% else %}Normal{% endif %}
         """, orderable=True, accessor='is_special', 
         verbose_name="Project type")
     project_code = tables.Column(accessor='code', verbose_name="Project 
            code")
  
     project_status = tables.Column(accessor='status', 
        verbose_name="Project status")
  
     department = tables.Column(accessor='department', 
        verbose_name="Department")
  
  
  
     class Meta:
        model = Project
        attrs = {'class': 'table table-striped table-hover'}
        sequence = (
          'project_name', 'project_type', 'project_code',
           'project_status',)
  </code></pre>
  
  <p>IN THE VIEW</p>
  
  <pre><code>from project.tables import ProjectTable
  from django_tables2.columns import CheckBoxColumn
  
  class AllocationChangeView(PagedFilteredTableView):
     display_message = "You need to be logged in to view this page"
     table_class = ProjectTable
     queryset = Project.objects.all()
     template_name = 'matter_allocation/change_project.html'
     paginate_by = ITEMS_PER_PAGE
     formhelper_class = ProjectFilterFormHelper
     filter_class = ProjectFilter
  
  def get_context_data(self, **kwargs):
      context = super(AllocationChangeView,
                      self).get_context_data(**kwargs)
      table = context['table']
      table.exclude = ('project_status','department')
      table.extra_columns =(('Change',CheckBoxColumn(checked=False)),)
      context['table'] = table
      return context
  </code></pre>