title: django-tables2 edit yesno parameter for all BooleanColumn
body: <p>I have a table.py where I would like to change the icons for True and False values of each BooleanColumn. I know that it can be modified by the yesno paramter of BooleanColumn, but I do not know how to override the default for all BooleanColumns. Here is the Code of tables.py (aacsb, amba, equis, mba, bsc, msc and doubedegree are BooleanFields):</p>
  
  <pre><code>from django_tables2 import Column, Table
  from manager.models import Partner
  
  
  class PartnerTable(Table):
  
      country_name = Column(accessor='country.name', verbose_name='Country')
      region_name = Column(accessor='country.region.name', verbose_name='Region')
  
      class Meta:
          model = Partner
          fields = ('name',
                    'country_name',
                    'region_name',
                    'website',
                    'aacsb',
                    'amba',
                    'equis',
                    'mba',
                    'bsc',
                    'msc',
                    'doubledegree',
                    )
  </code></pre>