Update base_list.html.twig

The additional blocks will allow adding attributes and additional classes to <table> and <th> tags without needed to make a copy the whole view. I'm interested in this because I'd like to use the FooTable.js library, which uses data attributes to set the auto-show/hide for each column (for responsive tables).

If it's of interest, I could create a pull request with the templating code that I'm adding into the <th> tags. It allows the attributes on the tag to be set per field by specifying as an option within the Admin class. I.e., 'th_attributes' => array('name' => 'value', ...)

Here's the additional code that could be included within the <th> tag: {% if field_description.options.th_attributes is defined %}{% for th_attribute, th_attribute_value in field_description.options.th_attributes %}{{ th_attribute }}={{th_attribute_value}}{% endfor %}{% endif %}
I think that the better name for the new block will be `sonata_list_table_class`. @rande what do you think?