enforce the use of goimports (#248)

* enforce the use of goimports

Instead of enforcing `gofmt`, enforce `goimports`, which also asserts
a separate section for non-builtin packages.

* run `goimports` everywhere

* exclude generated .pb.go files

config_test.go

@@ -1,12 +1,13 @@
 package nebula
 
 import (
-	"github.com/stretchr/testify/assert"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"testing"
 	"time"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestConfig_Load(t *testing.T) {