--help added as a proper option.

src/main.cr

@@ -37,6 +37,11 @@ OptionParser.parse! do |parser|
 		do_not_clean = true
 	}
 
+	parser.on("-h", "--help", "Prints this help message.") {
+		puts parser
+		exit 0
+	}
+
 	parser.invalid_option do |flag|
 		STDERR.puts "ERROR: #{flag} is not a valid option."
 		STDERR.puts parser