Support having a directory full of config files

Some programs like to have a directory of configuration files (arti.d?) and load all of the *.toml files from that directory. We could probably support this; we should figure out what interface we want first. (Open questions include: should there be a default directory? Should we support multiple directories? Should those configuration files be loaded before or after the default arti.toml? What should the command-line interface be?)