Made HostGroup.children a dictionary

ssh_config_utils/config_file.py

@@ -8,12 +8,12 @@ class HostGroup:
     def __init__(self, name):
         self.parent = None
         self.name = name
-        self.children = []
+        self.children = {}
 
     def add_child_group(self, name):
         child = HostGroup(name)
         child.parent = self
-        self.children.append(self)
+        self.children[name] = child
         return child
 
     @property