playlist: added extm3u plugin

This new plugin parses extm3u files.  Files without the "#EXTM3U"
header are still parsed by the plain old "m3u" plugin.

doc/user.xml

@@ -1337,6 +1337,14 @@ cd mpd-version</programlisting>
         </para>
       </section>
 
+      <section>
+        <title><varname>extm3u</varname></title>
+
+        <para>
+          Reads extended <filename>.m3u</filename> playlist files.
+        </para>
+      </section>
+
       <section>
         <title><varname>pls</varname></title>