Support creating store from a config file

[Wwwsylvia]

Proposed API:

package credentials

// StoreOptions provides options for NewStore.
type StoreOptions struct {
	// AllowPlainText allows saving credentials in plain text in configuration file.
	AllowPlainText bool
}

// NewStore returns a new store from the settings in the configuration
// file.
func NewStore(configPath string, opts StoreOptions) Store {
	panic("not implemented") // TODO: Implement
}

Design discussion: #18