Add options menu

[orhun]

Is your feature request related to a problem? Please describe.

It would be nice to have an options menu for tweaking the options (e.g. feed) inside the TUI.

We should also display the current feed etc. somewhere on the TUI. I haven't thought out the design yet.

Describe the solution you'd like

Add an options menu similar to kmon (popup in the center of the screen):

https://user-images.githubusercontent.com/24392180/76685907-4aed1d80-6628-11ea-96b7-a5bc0597455b.gif

Describe alternatives you've considered

None.

Additional context

Brainstorming is appreciated!