Better configuration (failure) handling

[ulfsauer0815]

Problems:

1. Configuration via environment variables
   The implementation is simple and relatively easy to deal with for deployment, docker etc., but maybe not as convenient and popular as file-based configuration.

2. Configuration loading is done in a simple Maybe monad, so there is no reporting on failures.
   Any kind of error in the configuration just yields an "invalid configuration" message and won't start.

Possible solutions:

• use a configuration library: probably file-based format (ini, json, yaml?), easy to deal with, built-in error handling
• use an something like ExceptT instead of `MaybeT'