Rename plugin_helpers to plugin-helpers (#31) (#33)

Signed-off-by: Joshua Li <joshuali925@gmail.com>
(cherry picked from commit 68d5009)

Co-authored-by: Joshua Li <joshuali925@gmail.com>

package.json

@@ -17,8 +17,8 @@
     "test:server": "plugin-helpers test:server",
     "test:browser": "plugin-helpers test:browser",
     "test:jest": "../../node_modules/.bin/jest --config test/jest.config.js",
-    "build": "yarn plugin_helpers build",
-    "plugin_helpers": "node ../../scripts/plugin_helpers"
+    "build": "yarn plugin-helpers build",
+    "plugin-helpers": "node ../../scripts/plugin_helpers"
   },
   "dependencies": {
     "brace": "0.11.1",