Added enable_systemd feature This feature makes systemd support optional, on by default. While it may seem strange that this feature exists, it makes sense for authors of applications who want to make systemd optional. Thanks to this feature the interface stays the same, it just fails to parse `systemd://` addresses with a helpful error message.

--- a/tests/systemd.rs	Sun Nov 29 14:15:33 2020 +0100
+++ b/tests/systemd.rs	Thu Dec 03 16:34:09 2020 +0100
@@ -23,7 +23,7 @@
 }
 
 #[test]
-#[cfg_attr(not(linux), should_panic)]
+#[cfg_attr(not(all(linux, feature = "enable_systemd")), should_panic)]
 fn main() {
     comm::main::<Test>();
 }