Feature/more css config

Added a few configuration options to allow user to specify CSS styles applied to paginator pages. Useful for e.g. styling pages using Pure CSS (which is what prompted me to do this).

The additional configurations options are relatively innocuous, but you might not like the fact that styles are added to the nested <a> tag in each <li> item; this is to accommodate some CSS libraries that append multiple classes to a single item, e.g. class="item item-active".