Show some "arc" help pages through a configurable pager, like "less"

Summary:
Fixes T5420. Some "arc help ..." is long and most similar commands send this kind of output through a pager.

Use a pager in at least some cases.

Test Plan: Ran "arc help land", got pager output. Ran "arc help land | cat", got raw output.

Maniphest Tasks: T5420

Differential Revision: https://secure.phabricator.com/D21327