Add a YAML config option

[caendesilva]

In addition, I would like to implement a YAML config option. This would be an optional feature, and work as a superset over the PHP configs. When the application boots, the YAML config is merged over the others. This, paired with a defined JSON schema would bring a stable location for project configuration settings. The PHP config files would still be used to configure behaviour, while the YAML one is for presentation.

Originally posted by @caendesilva in #230 (comment)

[caendesilva]

Going to take a stab at implementing this, and having it tied one-to-one with the new site config.

[caendesilva]

Having two config files doing the same thing adds a ton of complexity. You'd probably have to commit to using only one. Closing for now.