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 %}
Not really your fault, but the lines you modified are too long. Maybe you could set an example showing how this should be done. If you do, make sure you read [the § about whitespace control in the twig documentation](http://twig.sensiolabs.org/doc/templates.html#whitespace-control).