Add helper class for parsing pinterest settings from app settings

[TheFriendlyCoder]

Currently I have all of the parsing logic for the entire app settings file defined in a single class. I'd like to start defining a class hierarchy that allows subsections of the config to be parsed and analyzed separately from the core. To start with I'm thinking of defining a pattern for extracting service-specific settings from the rest of the file, starting with a section for Pinterest.

For now I'm thinking of defining a "get_service()" method that takes as input the name of a service and returns an object that manages the settings definition for the given service. In this way the framework will be extensible to new services as the need arises. This could also lay the foundation for making the settings file extensible via plugins as well.