Proper model definition of app configuration

[arnauorriols]

My understanding is that it reads a config file and merges it to Args. But I'm a little bit concerned with this approach because Args is just a parsed result of std's parse function, which doesn't have any domain knowledge of individual applications.

What I'd like to propose is to define a new class/object that stores all configurable values (which I'll call Config) and construct Config from the config file, command line args, and even env vars, then we'll be able to use the Config instance throughout the process.

Originally posted by @magurotuna in #193 (review)