Add custom handler to add inline feature flags

The changesets introduce custom handler for inline feature flags.

One notable difference is, instead of specifying feature flags in `dict`-style:

```python
FEATURE_FLAGS = {
    "finished": False,
}
```

the flag must be written in uppercased plain string with `FEATURE_FLAGS_X` where `X` is the feature name:

```python
FEATURE_FLAGS_FINISHED = False
```

The motivation behind this custom handler is to interopt with other Flask extensions, e.g. [Flask-AppConfig](https://pypi.python.org/pypi/flask-appconfig).