System and user level networks.yml

Let sb use system and user-level `networks.yml` file.  sb should search the following paths in order, using the first one it finds:

1) `$(pwd)/networks.yml`
2) `$HOME/.solidbyte/networks.yml`
3) `/etc/solidbyte/networks.yml`

Perhaps it should also merge them in reverse order, allowing pwd level settings to override user, which overrides system?