Add method to wrap existing bolt instance

[ivanjaros]

func Wrap(db *bbolt.DB, options *Options) (*Store, error) {
	options = fillOptions(options)
	return &Store{
		db:     db,
		encode: options.Encoder,
		decode: options.Decoder,
	}, nil
}

[tmm1]

This would be very useful. @ivanjaros perhaps you can wrap your code snippet in a PR?