Document and support the undocumented :value option

[benwbooth]

I'm using the :value option to pipe the output of each reply evaluation to a pager, i.e. less. This is useful for viewing large incanter datasets because now I can use the arrow keys to scroll around in two dimensions. It would be great if you could document the :value option. I think other people would find it useful too.

[trptcolin]

Yep, thanks for opening this. I'll likely change the name to something more descriptive like :value-printer, but first I need to wire up the --standalone mode to use this.