started moving settings to configuration file, starting with user index

using TOML, an INI-like format, because it's relatively user-friendly

`naveed.ini` is read from the application's root directory (a somewhat
muddy concept right now) by `main.go` (or manually within the test
suite), setting `Config.UserIndex` which is used to resolve users

note that the `userindex` package does not use that configuration to
avoid circular dependencies (note the inline comments; perhaps
cfa6bc845b079259086797bf86dde4a642c65879 should be reverted?)