Make explicit "mkcert -help" print to stdout (#265)

Currently "mkcert -help" prints to stderr, which is rather annoying as:

	$ mkcert -help | less

Gives us a blank page, as it pipes only stdout. To get any results in
less I need to use:

	$ mkcert 2>&1 | less
	$ mkcert |& less     # Non-standard bash/ zsh

Since the user explicitly asked for help with -help, it doesn't make
much sense to output it to stderr IMHO.