Add a helper method declare multiple key flags.

[dseomn]

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:

# foo.py
FEATURE1_KEY_FLAGS = ('foo', 'bar')
FEATURE2_KEY_FLAGS = ('quux', 'baz')

# main.py
flags.declare_key_flags(foo.FEATURE1_KEY_FLAGS)