implement check command

cmd/check/check.go

@@ -1,7 +1,34 @@
 package main
 
-import "fmt"
+import (
+	"encoding/json"
+	"os"
+
+	"github.com/concourse/github-release-resource"
+)
 
 func main() {
-	fmt.Println("[]")
+	var request resource.CheckRequest
+	inputRequest(&request)
+
+	github := resource.NewGitHubClient(request.Source)
+	command := resource.NewCheckCommand(github)
+	response, err := command.Run(request)
+	if err != nil {
+		resource.Fatal("running command", err)
+	}
+
+	outputResponse(response)
+}
+
+func inputRequest(request *resource.CheckRequest) {
+	if err := json.NewDecoder(os.Stdin).Decode(request); err != nil {
+		resource.Fatal("reading request from stdin", err)
+	}
+}
+
+func outputResponse(response []resource.Version) {
+	if err := json.NewEncoder(os.Stdout).Encode(response); err != nil {
+		resource.Fatal("writing response to stdout", err)
+	}
 }