[backport] add backport tool

.backportrc.json

@@ -0,0 +1,8 @@
+{
+  "upstream": "elastic/elasticsearch-js",
+  "branches": [
+    { "name": "15.x", "checked": true },
+    "14.x"
+  ],
+  "labels": ["backport"]
+}

package.json

@@ -58,6 +58,7 @@
     "async": "~0.8.0",
     "babel-eslint": "^8.2.2",
     "babel-loader": "^8.0.0-beta",
+    "backport": "^3.0.2",
     "blanket": "^1.2.3",
     "bluebird": "^2.9.14",
     "browserify": "^16.1.1",

scripts/backport.js

@@ -0,0 +1 @@
+require('backport');