Add config file support

Currently, options can only be specified as command line arguments. This will get tedious when integrating the formatter into editors.

`yangfmt` should look in the following locations for configuration:

- `./yangfmt.toml`
- `~/.config/yangfmt.toml`
- `~/.config/yangfmt/yangfmt.toml`

Relevant libraries:
- https://github.com/dirs-dev/directories-rs
- https://github.com/mehcode/config-rs