Added modal auto-closing on Settings Submit

[gderiyenko]

This PR fixes the issue #45

[gderiyenko]

Created new component ./widgets/notification.js, since it is used in index.html and in options.html.

In order to pass notification message through a page reload, I used localStorage [pendingNotification].

On page initialization: 1. check for pending notification, 2. flush the message

[ujain1999]

I agree with this change but I think the code for notifications should not live in the widgets folder. In the current scope of the project, widgets are the modules that appear in the sidebar. Move the code for notifications to main.js

[BaldrianSector]

Should this wait till after the #52 refactor @ujain1999 to avoid merge conflicts?