Adding tabs

[glvr182]

Describe the feature you'd like
Sometime there is just not enough room in the terminal, or two views are
closely related. This calls for a new thing called a "tab". Just like with your browser, a tab holds different information but does not require you to start up a new window.

Describe alternatives you've considered
Manual tabbing, but it is a pain for the developer to create this each and every time.

Additional context
The title of the view would look like this: ─Tab one / Tab two─. and the current tab would be highlighted.

[mjarkk]

The idea of tabs is amazing thought ─Tab one / Tab two─ looks a bit off to me.
I'm also wondering how this would look on small screens when having more than 2 tabs: ─Tab one/T/T/T─.

[glvr182]

Agreed, I still have to find a good way of displaying tabs. This was my first thought. I think when terminals are too small to display a title we should cut it per tab. So lets say it is first First / Second it would go to
F / S

[vladkras]

Or maybe
┌─| First | Second |──

┌─ First | Second ──

[mjarkk]

@vladkras I think it's a bad idea to use | because it's the same character as the box frame.
I think this could be confusing when the terminal gets too small or overlap because the | might look like a border.

[glvr182]

Picking this up again after #41
Will integrate the following commits:
jesseduffield@619845f
jesseduffield@3219759
jesseduffield@117e4fb