Add expenses from file

Allow to upload a JSON file containing a list of expense items

app/utils/read-file-content.js

@@ -0,0 +1,8 @@
+export default function (file) {
+  const reader = new FileReader();
+  return new Promise((resolve, reject) => {
+    reader.onload = event => resolve(event.target.result)
+    reader.onerror = error => reject(error)
+    reader.readAsText(file)
+  })
+}