Instead of reading our own TOML file for configuration, we can hook into
Rocket's [built-in configuration system][0]. Although it doesn't matter
much right now, it may if we end up using Rocket's SQL [database
integration][1], because it uses the same mechanism. Without making
this change, we would end up with two configuration files in that case.
[0]: https://rocket.rs/guide/v0.5/configuration/#extracting-values
[1]: https://rocket.rs/guide/v0.5/state/#databases
Dustin