Add feature flags

[roobottom]

In the MYA prototype, we have a flags system that allows any user to turn prototype features on or off. This is especially useful when testing different variants, such as different content or different journeys.

We can set a flag (currently limited to bools) via

• env variables
• via GET, through query string parameters, e.g., ?feature_name=true&feature_othername=off (on/off and true/false supported)
• via POST through a form
• a defaults.js file

Variables are persisted in the session and are available via

• session.data.features[var_name]
• req.features[var_name] for routing

Might this be useful for others?