New translations and auto-save changed options

• Add new translations
• Update preference on change instead of submit/save
• Update icon on all tabs when icon option is changed
• Increase icon sizes

Make async & add option to control opening feeds

Add option in settings to control opening feeds in:

• Current tab
• New tab
• New window

List of significant changes made

• Add openFeed to options.html
• Control opening feeds based on this preference
• Make all Promise handling use async/await

Add options page to pick icon

• Add Form for options page
• Add storage permission (required to save settings)
• Add JavaScript to update storage on options form submission
• Add options_ui
• Add ability to choose from 3 icons

Update subscriptions on navigation

• Add webNavigation permission (required to watch for navigation) bug #25
• Remove activeTab permission
• Create tab for feed to prevent opening in window
• Update screenshot

Construct using `<template>`s

• Rewrite to use <template>s to make maintaining easier
• Add slight animation on popup

Translation (cs), messaging, & contributor

• Make localizable and add Czech (cs) translation
• Reduce unnecessary messaging
• Add @MikkCZ MikkCZ as contributor

Match popup style

• Update popup to match Firefox popups style
• Update all tabs on first load

Special thanks to @MikkCZ

Handle multiple subscription options

• Add HTML document for popup
• Construct feeds by passing JSON in URLSearchParams
• Add Content-Security-Policy, only allowing 'self'
• Improve build script by updating ignore param
• Fix link to AMO users in README
• Add Gitter notifications in Travis-CI config

Update icons

• Use subscribe icon from Firefox Icons
• Add dark theme variant

Fix missing tab data

Was not storing tab info, so attempts to open feed
for tab would fail.