* Read ImporterConfiguration from .toml file if present. Also check for presence of dependency programs.
* Actually use config.osmconvert instead of hard-coded 'osmconvert'
* Add curl to checked dependencies
* Don't require all fields to be present in importer.toml configuration. Fill in defaults.
* Remove unneeded slice
* Use json for importer config and use serde(default) attribute to simplify application of defaults
* Extract re_dependencies_callable from main.rs and remove dependency on Job
* Revert change to abstutil, use struct-level default attribute
* Add importer.json to .gitignore
* Make gunzip_args string instead of Option<string>
as part of this, wound up grabbing fresh OSM and GTFS. but actually cant
reimport OSM right now, because fixes reference old OSM IDs and things
break. so sticking with old OSM, but new GTFS. urgh.
