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.
Merge branch 'abjerner-patch-1' into 6.2.0

Conflicts:
	src/Umbraco.Web/PublishedContentExtensions.cs