support other SQLite wrappers, and various hooks needed by grist-static (#516)

app/client/DefaultHooks.ts

@@ -0,0 +1,11 @@
+import { UrlTweaks } from 'app/common/gristUrls';
+
+export interface IHooks {
+  iframeAttributes?: Record<string, any>,
+  fetch?: typeof fetch,
+  baseURI?: string,
+  urlTweaks?: UrlTweaks,
+}
+
+export const defaultHooks: IHooks = {
+};