API extension points (#473)

* API endpoint extensions working.

extensions example.

* Added server.NewEnv and some docs for the API extensions example.

extensions example.
example main.go.

* Uncommented special handler stuff.

* Added section in docs for extending API linking to example main.go.

* Commented out special_handler test

* Changed to NewFromEnv

main.go

@@ -2,31 +2,14 @@ package main
 
 import (
 	"context"
-	"os"
 
-	log "github.com/Sirupsen/logrus"
-	"github.com/iron-io/functions/api/datastore"
-	"github.com/iron-io/functions/api/mqs"
 	"github.com/iron-io/functions/api/server"
-	"github.com/spf13/viper"
 )
 
 func main() {
-	ctx := contextWithSignal(context.Background(), os.Interrupt)
+	ctx := context.Background()
 
-	ds, err := datastore.New(viper.GetString(server.EnvDBURL))
-	if err != nil {
-		log.WithError(err).Fatalln("Invalid DB url.")
-	}
-
-	mq, err := mqs.New(viper.GetString(server.EnvMQURL))
-	if err != nil {
-		log.WithError(err).Fatal("Error on init MQ")
-	}
-
-	apiURL := viper.GetString(server.EnvAPIURL)
-
-	funcServer := server.New(ctx, ds, mq, apiURL)
+	funcServer := server.NewFromEnv(ctx)
 	// Setup your custom extensions, listeners, etc here
 	funcServer.Start(ctx)
 }