Support j/k (vim keys) for navigation

Hey,

I would love to have a way to go up/down tasks using j/k (the keys used for vertical navigation in vim). This would let me keep my keys on the home row more instead of having to reach for the arrow keys when I want to go to a different task.

We could also use h/l for horizontal navigation, perhaps as an alternative for ( and ) to jump between tabs.

Keep up the good work, I'm loving the product!

Regards, Grzegorz