Adding Answer request AND adding best_of option to Completion. (#5)

* adding best of option

* adding answers option

* fixing small mistake

completion.go

@@ -25,6 +25,7 @@ type CompletionRequest struct {
 
 	PresencePenalty  float32 `json:"presence_penalty,omitempty"`
 	FrequencyPenalty float32 `json:"frequency_penalty,omitempty"`
+	BestOf           int     `json:"best_of,omitempty"`
 }
 
 // Choice represents one of possible completions