Add custom handler to add inline feature flags

[iromli]

The changesets introduce custom handler for inline feature flags.

One notable difference is, instead of specifying feature flags in dict-style:

FEATURE_FLAGS = {
    "finished": False,
}

the flag must be written in uppercased plain string with FEATURE_FLAGS_X where X is the feature name:

FEATURE_FLAGS_FINISHED = False

The motivation behind this custom handler is to interopt with other Flask extensions, e.g. Flask-AppConfig.

[catermelon]

Awesome, thank you so much for the contribution. Did you have anything else you wanted to add? Otherwise I'll release a new version today or tomorrow.

[iromli]

Nothing at the moment. Thanks @trustrachel