Add support for stored application settings

[TheFriendlyCoder]

Before I can go much further adding features to the app, I need to develop a pattern for storing and retrieving application settings to disk so that I can save user configurable parameters like connection params for various services. I'm thinking for now of just storing all the user configurable settings in a single config file formatted as YAML, and stored in the current user's home folder.

[TheFriendlyCoder]

I should make use of the appdirs library to make it easier to implement a cross-platform compatible solution for storing this configuration data.