trampoline configurations

[mcandre]

Enrich the configuration system, so that child configuration files inherit and override settings from configuration files in ancestor folders, similar to how git processes .gitignore files.

Trampoline style configuration loading is essential for resolving many configuration system UX quirks:

• Supporting a convenient home directory default configuration (~/todolint.toml), while still allowing for a subset of projects to override with their own todolint.toml files
• Referencing file paths not directly in the current working directory
• Interacting with mixed-language projects (e.g. English + Spanish)