Allow to switch the header off in `CommandLineOptionFormatter`.

This pull request just adds the option to turn the header off. The default behavior is unchanged.

But actually I think that the output of the header does not belong in to the default options formatter. 

The first time I saw the header in the console output, I found it quite confusing because of its similarity to an actual option. May be I am somewhat stupid, but I always thought that there is another (buggy?) option named "/Value" where the description is missing and "Description" is shown as a default :). An empty line below the header would be nice, but I personally would prefer it being switched off by default.

Also the default formatter should not be public or - if intended to be used - not in a namespace that includes "Internals". Of course people could always write their own, but it's almost preferable to configure what's already there.