Optional user-definable adapter for DEFAULT_FLAG

[lmeyerov]

Currently, DEFAULT_FLAG is a global default for new flags during WAFFLE_CREATE_MISSING_FLAGS, but that breaks down as soon as 2 flags should have different defaults, or any other late/contextual programmatic determination

One solution is somehow tying this to SETTINGs, as in #50

A more general and Django-y approach would be allowing the specification of a user-defined handler like an Adapter. Ex:

DEFAULT_FLAG_ADAPTER='my.waffle.my_flag_adapter'
...
class my_flag_adapter(waffle_core_flag_adapter):
    def resolve_flag_default(flag: str) -> bool:
      try:
          return config[f'FLAG_{flag};]
      except:
          1
      if flag == 'special_flag_1':
         return True
      return config['DEFAULT_FLAG']

A more static and closed approach would be constance-style precomputed settings:

WAFFLE_FLAG_DEFAULTS = OrderedDict([
  ('flag_x', (env.bool('FLAG_X', default=False))),
  ...
])

[mscansian]

@lmeyerov The way that it can be mitigated right now is to invert the condition of the flag so that the desired default is always False. Instead of IS_SOMETHING_ACTIVE, flip it to IS_SOMETHING_INACTIVE. But this is annoying. Having an option to set different defaults would be nice.