Command to check if number is registered (#391)

* Added the isRegistered command

* Minor fixes

* Corrected description

* Added AsamK's suggestions

Fixes #178

src/main/java/org/asamk/signal/commands/Commands.java

@@ -11,6 +11,7 @@ public class Commands {
         addCommand("addDevice", new AddDeviceCommand());
         addCommand("block", new BlockCommand());
         addCommand("daemon", new DaemonCommand());
+        addCommand("getUserStatus", new GetUserStatusCommand());
         addCommand("link", new LinkCommand());
         addCommand("listContacts", new ListContactsCommand());
         addCommand("listDevices", new ListDevicesCommand());