Configuration loading overhaul

[Aetf]

• Fix a small issue in test about XDG_RUNTIME_DIR handling
• Simple merging logic for Config
• Load config from system-wide locations (fixes #30)
• Dynamically pick up changes from newly created config files (fixes #29)
• Use directories-rs to find the user-level config directory path (We should also consider using it for XDG_RUNTIME_DIR later)

Please see the commit message for a more detailed description of the change.

@ethanpailes currently, the config merging rule is very simple: higher-priority top-level keys always fully overwrite lower-priority ones. However we should probably handle arrays and maps differently. Let me know what you think is the best strategy.

Also the PR is fairly large, let me know how I can make it easier to review.

[Aetf]

Sure, just force pushed to fix all those.

Edit: not quite :/ will make sure fixing lints before merge.

Thank you for the detailed review!