Use to_css_class for table cell column classes

This ensures that columns with spaces in the name will still
generate usable CSS class names. Refs #209

datasette/templates/_rows_and_columns.html

@@ -20,7 +20,7 @@
     {% for row in display_rows %}
         <tr>
             {% for cell in row %}
-                <td class="col-{{ cell.column|lower }}">{{ cell.value }}</td>
+                <td class="col-{{ cell.column|to_css_class }}">{{ cell.value }}</td>
             {% endfor %}
         </tr>
     {% endfor %}