Add defaultPager option

[sumbach]

See also denysdovhan/bash-handbook#46.

This PR adds a defaultPager option (perhaps we could give this a better, less ambiguous name) to allow overriding the default of more. The difference between defaultPager and pager is that pager takes precedence over the $PAGER environment variable, whereas defaultPager is only used when $PAGER is not set.

[eush77]

I think having both pager and defaultPager is very confusing. Can you use something like

pager({ pager: process.env.PAGER || 'default-pager' })

instead?

[sumbach]

@eush77 Good call; I ended up setting default values for PAGER and LESS (to match git behavior) instead of making a change to default-pager: https://github.com/denysdovhan/bash-handbook/pull/46/files