Provide a function to easily return `Hosts`

Currently we have to manually decode the configuration file:

```golang
f, _ := os.Open(filepath.Join(os.Getenv("HOME"), ".ssh", "config"))
cfg, _ := ssh_config.Decode(f)
// cfg.Hosts
```

It'd be nice to have a function like `ssh_config.GetHosts()`. It should defaults to `DefaultUserSettings` unless overridden the config path from the parameters.