Add a helper method declare multiple key flags.

Would it be possible to get a new function `declare_key_flags()` that behaves like `declare_key_flag()` but takes a Collection of flags instead of a single flag? It would make it easier to do something like this:

```python
# foo.py
FEATURE1_KEY_FLAGS = ('foo', 'bar')
FEATURE2_KEY_FLAGS = ('quux', 'baz')

# main.py
flags.declare_key_flags(foo.FEATURE1_KEY_FLAGS)
```