add support for loading .pls playlists

only through "Add" prompt since listplaylists command doesn't
list .pls playlists present in mpd playlists directory.

src/ncmpcpp.cpp

@@ -1254,7 +1254,10 @@ int main(int argc, char *argv[])
 				else
 				{
 					static const char lastfm_url[] = "lastfm://";
-					if (path.compare(0, static_strlen(lastfm_url), lastfm_url) == 0)
+					if (path.compare(0, static_strlen(lastfm_url), lastfm_url) == 0
+					||  (path.find('/') == std::string::npos
+					&&   path.find(".pls", path.length()-4) != std::string::npos)
+					   )
 						Mpd.LoadPlaylist(path);
 					else
 						Mpd.Add(path);