Add UMD module for direct browser import

Author: alessiomaffeis

poi.config.js

@@ -0,0 +1,12 @@
+module.exports = {
+  entry: './PictureInput.vue',
+  filename: {
+    js: 'vue-picture-input.js'
+  },
+  moduleName: 'PictureInput',
+  extractCSS: false,
+  dist: 'umd',
+  sourceMap: false,
+  html: false,
+  format: 'umd'
+}