Explicitly paginate just help text instead of all output

Before we tried to automatically paginate all `puts` output depending on
whether it fits on one page using LESS options. This didn't work as
intended, so now explicitly opt in for pagination for `hub help -p` and
`hub help hub` output.

Doing so enables us to clear the screen reliably upon exiting the pager
without risking single-page output being swallowed.

This reverts commit 45003e19524dafa7dbfe23685eb0e7010b223957.

Fixes #368
Don't allow man page width to exceed 120 characters

Text wider that that is hard to read. Also, on terminals narrower than
120 chars, provide a 1-character right margin to improve readability in
case of vertical splits.
Show `hub <command> -h` short usage help on stdout

Instead of stderr, show this help on stdout because we suppose that the
user intended to retrieve help intentionally.
Merge branch 'terminal-tweaks'