Add a Command to Reload a Config File

[oneirocosm]

Currently, UserSettings.loadConfig has a type of sync.Once so the config files only need to be reloaded once. This can be inconvenient if the files need to be re-parsed for updated values in the future.

As discussed here, by making this a pointer to sync.Once, and adding a helper function, it would be possible to reload the configuration when necessary.

[giraffesyo]

You can achieve this right now by doing:

ssh_config.DefaultUserSettings = &ssh_config.UserSettings{}

This works because the code checks for nil values of userConfigFinder and systemConfigFinder

https://github.com/kevinburke/ssh_config/blob/master/config.go#L288-L289
https://github.com/kevinburke/ssh_config/blob/master/config.go#L299-L300