Utilize Nickel for runtime configuration

[MagicRB]

Utilizing Nickel for runtime configuration is an alternative to template style configuration, like go-template, django or embedded EEx. This would allow us to use typed schemas for configuration files, while also tightly configuring what the templates have access to. This is related to #3, as that issue gave as an example envsubst.

Access Control

This could be configured at build time with Nix, to make sure that the template system for a particular service can't leak information from other services or other critical files.