v0.5.1 release

• Made OAuth2 AppFlow run local server and open in browser when using GUI. This should entirely rid us of needing access to a console \o/
• Made application a bit more robust when tackling OAuth failures, be it aborted auth or missing files.
• Moved API Key/OAuth file exists logic to MainWindow and made it a pop-up. In the case of a missing key or client secret file, the user will from now on be prompted from GUI about whether they want to load their own or use the public set (NB: prone to exceeded quota).
• Made it possible again to run CLI commands without GUI, using the --no_gui flag.
• Added option for doing a single refresh and print of subfeed with --refresh_and_print_subfeed
• Made all icon constant variables in MainWindow default to the light version, to avoid false positive "NoneType" warnings in code.
• Changed Configuration pop-out window to a CentralWidget/View and added it to toolbar.
• Renamed all instances of "preferences" (both in code and files) to "config" to make it less confusing as the class itself is called ConfigView.
• Added --print_playlist_items cli option with and optional --print_playlist_items_url_only switch for use in exploding YouTube playlist IDs into individual video items.
• Fixed long lasting "Too many open files" bug caused by thread-unsafe google-api-client httplib2.