Add the ability to print a subtree in KCFG

[palinatolmach]

As suggested by @lisandrasilva, it'd be easier to debug proofs if kontrol show allowed printing a subset of nodes between two different nodes.

[ehildenb]

We could add kontrol show --focus ... which you give a node-id, and it will print the path from th eroot to that node only. We could also have the f keystroke (for "focus") in the kontrol view ... hide the other paths and only display that one.

We could also add teh z keystroke to the kontrol view ... to make the behavior view atke the whole screen, instead of the left half.

[lisandrasilva]

Both would have been super useful in the Term engagement, where we had pretty big kcfgs.