Implement configuration file support with config-rs
- Add config and toml dependencies
- Create layered config system: Defaults > Config Files > ENV > CLI
- Support config loading from multiple paths:
- ~/.config/exp2span/config.toml
- ~/.exp2span.toml
- /etc/exp2span/config.toml
- .exp2span.toml
- Custom path via --config flag
- Environment variable support with EXP2SPAN prefix
- CLI args override config and environment variables
- Merge config with CLI arguments for final configuration
Resolves exs-njr