Option to ignore undefined tags

Would it be possible to have an option to ignore undefined tags instead of raising a `yaml.constructor.ConstructorError`? 

Something like:
```
def construct_undefined(self, node):
    if ignore_undefined:
         return None
    raise ConstructorError(None, None,
            "could not determine a constructor for the tag %r" % node.tag,
            node.start_mark)
```