[commands] Add Command.qualified_name to get the full command name.

This also sets `__str__` to do the same thing.

discord/ext/commands/core.py

@@ -306,6 +306,38 @@ class Command:
 
         return result
 
+    @property
+    def full_parent_name(self):
+        """Retrieves the fully qualified parent command name.
+
+        This the base command name required to execute it. For example,
+        in ``?one two three`` the parent name would be ``one two``.
+        """
+        entries = []
+        command = self
+        while command.parent is not None:
+            command = command.parent
+            entries.append(command.name)
+
+        return ' '.join(reversed(entries))
+
+    @property
+    def qualified_name(self):
+        """Retrieves the fully qualified command name.
+
+        This is the full parent name with the command name as well.
+        For example, in ``?one two three`` the qualified name would be
+        ``one two three``.
+        """
+
+        parent = self.full_parent_name
+        if parent:
+            return parent + ' ' + self.name
+        else:
+            return self.name
+
+    def __str__(self):
+        return self.qualified_name
 
     @asyncio.coroutine
     def _parse_arguments(self, ctx):