Package fflag provides a simple feature flag system.
Feature names are lowercase and can only contain letters, numbers, undercores
and dots.
good: "foo", "foo_bar", "foo.bar"
bad: "Foo", "foo-bar"
A feature flag can be enabled by the user with an environment variable
or by adding it to {ConfigDir}/feature.yaml
I.e. CROWDSEC_FEATURE_FOO_BAR=true
or in feature.yaml:
```
---
- foo_bar
```
If the variable is set to false, the feature can still be enabled
in feature.yaml. Features cannot be disabled in the file.
A feature flag can be deprecated or retired. A deprecated feature flag is
still accepted but a warning is logged. A retired feature flag is ignored
and an error is logged.
A specific deprecation message is used to inform the user of the behavior
that has been decided when the flag is/was finally retired.
