Add support for env var instead of url option?

[mguinness]

As an alternative to passing the -u option to the amqp-tools could an environment variable like amqp_url be read with getenv() if defined?

rabbitmq-c/tools/common.c

Lines 142 to 144 in 8b7471e

	struct poptOption connect_options[] = {
	{"url", 'u', POPT_ARG_STRING, &amqp_url, 0, "the AMQP URL to connect to",
	"amqp://..."},