Overloads for HumanizedLabels.Register to support all Humanizer letter casings

I saw in the doco that you could use some of the other Humanizer letter casing strategies by copying the HumanizeLabels class into your project and changing the selected enum. I thought it would be nicer to pass the LetterCasing enum value you want to the Register method. I used a default argument value of LetterCasing.Sentence so that the existing API and behaviour are not changed.