Add feature zeroconf in order to support avahi mDNS registration via zeroconf crate #115

Job	Run time
build_and_test (rustcrypto, stable)	1m 26s
build_and_test (rustcrypto, nightly)	1m 39s
build_and_test (rustcrypto, alloc, stable)	1m 36s
build_and_test (rustcrypto, alloc, nightly)	1m 38s
build_and_test (rustcrypto, os, stable)	2m 30s
build_and_test (rustcrypto, os, nightly)	2m 25s
build_and_test (mbedtls, stable)	1m 55s
build_and_test (mbedtls, nightly)	1m 39s
build_and_test (mbedtls, alloc, stable)	1m 46s
build_and_test (mbedtls, alloc, nightly)	1m 57s
build_and_test (mbedtls, os, stable)	2m 45s
build_and_test (mbedtls, os, nightly)	2m 45s
build_and_test (openssl, stable)	1m 9s
build_and_test (openssl, nightly)	1m 25s
build_and_test (openssl, alloc, stable)	1m 30s
build_and_test (openssl, alloc, nightly)	1m 11s
build_and_test (openssl, os, stable)	2m 29s
build_and_test (openssl, os, nightly)	2m 45s
	34m 30s

Provide feedback