Resolve "Dynamically set envrionment variables"

Closes #27 (closed)

src/assets/env.template.js

+(function (window) {
+  window["env"] = window["env"] || {};
+
+  // Environment variables
+  window["env"]["apiUrl"] = "${API_BASE_URL}";
+  window["env"]["s3Url"] = "${S3_URL}";
+})(this);