Add commands for creating and updating view box about hired

[sidju]

The view box has been coded in unconditionally, printing from the start of selection - 5 to the height of the terminal window or end of the buffer, whichever comes first.
Some commands to modify this behavior would be good features.

[sidju]

Intended commands are:
P: Toggle printing the viewbox
N: Toggle numbering in the viewbox
L: Toggle printing "literally" in the viewbox

If people complain, eventually add a config flag or something for disabling the viewbox, but why then use this over 'ed'?

[sidju]

Viewbox is being retired in favor of a separate TUI frontend. Coming soon-ish.