Add ExtensionStore (from @polkadot/extension) (#142)

* Add ExtensionStore (from @polkadot/extension)

* Consistent naming, untested/unused File store

* Update File.ts

packages/ui-keyring/src/stores/Browser.ts

@@ -0,0 +1,29 @@
+// Copyright 2017-2019 @polkadot/ui-keyring authors & contributors
+// This software may be modified and distributed under the terms
+// of the Apache-2.0 license. See the LICENSE file for details.
+
+import { KeyringStore } from '../types';
+
+import store from 'store';
+
+export default class BrowserStore implements KeyringStore {
+  all (cb: (key: string, value: any) => void): void {
+    store.each((value: any, key: string) =>
+      cb(key, value)
+    );
+  }
+
+  get (key: string, cb: (value: any) => void): void {
+    cb(store.get(key));
+  }
+
+  remove (key: string, cb?: () => void): void {
+    store.remove(key);
+    cb && cb();
+  }
+
+  set (key: string, value: any, cb?: () => void): void {
+    store.set(key, value);
+    cb && cb();
+  }
+}