Support for DB index management

v0.8.3 introduces support for custom user-definable indexes through a simple API in the system/db package.

func Index(namespace, key string) ([]byte, error) {}  // getter
func SetIndex(namespace, key string, value interface{}) error {} // setter
func DeleteIndex(namespace, key string) error {} // delete key in index
func DropIndex(namespace string) error {} // remove index and all key/values

To get the latest version:

$ go1.8rc3 get -u github.com/ponzu-cms/ponzu/...
$ cd $PROJ && ponzu upgrade