Add current page class to html produced by wp_link_pages() function

[egorpromo]

Function wp_link_pages() makes row links like:

<a href="http://example/?p=1">1</a>  <a href="http://example/?p=1?page=2">2</a> <a href="http://example/?p=1?page=3">3</a>

In this case it is not obvious what page is current. Maybe it is needed to add some class to current page. Suppose current page is 2. In this case link for page 2 has class and html must be something like:

<a href="http://example/?p=1">1</a>  <a href="http://example/?p=1?page=2" class="current-page">2</a> <a href="http://example/?p=1?page=3">3</a>

So it is better for design and page usage.

#13578