Add xenv -U argument

With -U the "module use" command can be executed as part of the xenv evaluation chain.

xenv/modules.py

@@ -81,6 +81,11 @@ def purge(env):
 	return _module_command_function_pointer(env)(env, ["purge"])
 
 
-# Restore moduls from collection.
+# Restore modules from collection.
 def restore(env, name):
 	return _module_command_function_pointer(env)(env, ["restore", name])
+
+
+# Restore modules from collection.
+def use(env, modules):
+	return _module_command_function_pointer(env)(env, ["use"] + modules)