Update PublishedContentExtensions.cs

Added overloaded extension methods for Next() and Previous() to take a delegate (eg. lambda expression) as a second parameter. The methods are based on this thread by Jeavon Leopold: http://our.umbraco.org/forum/developers/razor/46832-Filtered-Next()-and-Previous()-v7-UmbracoHelperMVC

The code is tested in 7.0.0, but since it only relies on the existing Next() and Previous() methods, it should work in 6.1.x and 7.1.1 as well.