Provide option to customize header anchor links

[uzluisf]

Currently for the anchor links, the casing of headers is left untouched and spaces are substituted with an underscore. For instance, the header Signature of a Subroutine becomes #Signature_of_a_Subroutine. This is mostly personal but it'd look better if the words in an anchor link were in lowercase and interweaved with dashes instead (e.g., #signature-of-a-subroutine).