Allow "quiet" recipes prefixed with "@"

[casey]

Noticed this in the clap-rs justfile:

update-contributors:
	@echo 'Removing old CONTRIBUTORS.md'
	@mv CONTRIBUTORS.md CONTRIBUTORS.md.bak
	@echo 'Downloading a list of new contributors'
	@echo "The following is a list of contributors in alphabetical order:" > CONTRIBUTORS.md
	@echo "" >> CONTRIBUTORS.md
	@echo "" >> CONTRIBUTORS.md
	@githubcontrib --owner kbknapp --repo clap-rs --sha master --cols 6 --format md --showlogin true --sortBy login >> CONTRIBUTORS.md
	@rm CONTRIBUTORS.md.bak

I've done the same thing myself a few times, so it might be nice to mark a recipe as quiet, and then flip the meaning of "@" in that recipe:

@update-contributors:
	echo 'Removing old CONTRIBUTORS.md'
	mv CONTRIBUTORS.md CONTRIBUTORS.md.bak
	echo 'Downloading a list of new contributors'
	echo "The following is a list of contributors in alphabetical order:" > CONTRIBUTORS.md
	echo "" >> CONTRIBUTORS.md
	echo "" >> CONTRIBUTORS.md
	githubcontrib --owner kbknapp --repo clap-rs --sha master --cols 6 --format md --showlogin true --sortBy login >> CONTRIBUTORS.md
	rm CONTRIBUTORS.md.bak

Not sure if this is a good idea or not. It's not hard to prefix all lines of a recipe with "@".