Support persistent configuration

[mafik]

Many other features in Gatekeeper would need to store some state somewhere. This issue is about adding such mechanism. It should be:

• familiar (don't invent any new mechanisms or external dependencies)
• easy to debug by end-users
• forwards compatible

From the point of Gatekeeper it should support just two methods: Get(key) => value & Set(key, value).

It's ok for the config to be read on start-up only because any changes made by the user would involve restarting / reloading Gatekeeper. To support config reloading, two other features may be implemented as well:

1. Config validation
2. Config reloading without service restart

This mechanism may be based on EnvironmentFile= in systemd.