Add a horizontal tab bar for `boo ui`

As young as this project is it really sounds great!

One thing for me though is that I prefer a horizontal tab line. Main reason being that with a standard 4k monitor I use half of my screen for my terminal so I can have something else open at the safe time and so my terminal ends up in portrait mode and horizontal space is at a premium. If boo had a horizonal tab bar as an option it would basically match my current tmux use .

Thanks and keep up the good work!