add alternative config directories
add alternative config directories

Introduce new folders where the config file can live:
- current directory (old behaviour)
- user's settings directory (~/.config/ on Unix, %appdata% on Windows)
- system settings (/etc/ on Unix)

The first one found is used; in other words, having a local config file
overrides a global one.

greatwolf/blackbird,greatwolf/blackbird,butor/blackbird,butor/blackbird,greatwolf/blackbird,butor/blackbird