chore: add npm publish job to circle ci

.circleci/config.yml

@@ -6,6 +6,9 @@ test_fixtures_cache_key: &test_fixtures_cache_key test-fixtures-cache-{{ .Enviro
 test_env: &test_env
   GCLOUD_PROJECT: 0
   GCLOUD_TRACE_NEW_CONTEXT: 1
+release_tags: &release_tags
+  tags:
+    only: /^v\d+(\.\d+){2}(-.*)?$/
 
 # Unit test workflow.
 
@@ -77,10 +80,24 @@ workflows:
   version: 2
   tests:
     jobs:
-      - node4
-      - node6
-      - node8
-      - node9
+      - node4:
+          filters: *release_tags
+      - node6:
+          filters: *release_tags
+      - node8:
+          filters: *release_tags
+      - node9:
+          filters: *release_tags
+      - publish_npm:
+          requires:
+            - node4
+            - node6
+            - node8
+            - node9
+          filters:
+            branches:
+              ignore: /.*/
+            <<: *release_tags
 
 jobs:
   node4:
@@ -119,3 +136,18 @@ jobs:
       - *postgres_service
       - *mysql_service
     <<: *unit_tests
+  publish_npm:
+    docker:
+      - image: node:8
+        user: node
+    steps:
+      - checkout
+      - run:
+          name: Set NPM authentication.
+          command: echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > ~/.npmrc
+      - run:
+          name: Install modules and dependencies.
+          command: npm install
+      - run:
+          name: Publish the module to npm.
+          command: npm publish