add simple form

src/settings/index.js

@@ -0,0 +1,19 @@
+import './settings.scss';
+
+let form = document.getElementById('vimvixen-settings-form');
+
+form.addEventListener('submit', (e) => {
+  let value = {
+    json: e.target.elements['plain-json'].value
+  };
+  e.preventDefault();
+  browser.storage.local.set(value);
+});
+
+document.addEventListener('DOMContentLoaded', () => {
+  browser.storage.local.get().then((value) => {
+    if (value.json) {
+      form.elements['plain-json'].value = value.json;
+    }
+  }, console.error);
+});