Improve user control of paging

[jdbaldry]

This allows users to avoid a pager by emptying out the PAGER environment variable (setting PAGER=""), only defaulting to less with posix flags (not supported by busybox less) if PAGER is unset.

Closes #293

[jdbaldry]

I haven't yet added a top level --no-pager option but would be willing to make that change if desirable. Presumably it would be a top level option that would globally disable paging for all subcommands.

[jdbaldry]

I've preferred not breaking/changing the existing default pager behavior over supporting the simplest pager but that may be worth reconsidering if we think running in a busybox environment is more likely.

Also, I'm not sure how paging works on windows but this probably doesn't make the situation any worse. Possibly Windows has the more command (https://superuser.com/questions/426226/less-or-more-in-windows/426229) but I don't have any convenient environments in which to check this.

[sh0rez]

Can't print an io.Reader, need to read first :)

Maybe put this the bottom of the func:

_, err := io.Copy(os.Stdout, r)
return err

Then also when the cmd fails, don't return but instead just do "nothing", so it automatically falls back to above code

[jdbaldry]

Whoops! The proposed refactor sounds nice but changes the function signature for fPageln and potentially for pageln too. If that's cool, I'll go ahead and make this change.

[sh0rez]

I'm happy with both fPageln and pageln returning errors, it should be up to the consumer to ignore them if applicable.

Also makes the control flow inside a bit simpler I guess

[jdbaldry]

Thanks for all the feedback @sh0rez. I think this is ready for a second pass now :)

[sh0rez]

Perfect!!

Have some nitpicks, feel free to ignore

[sh0rez]

If it's the last statement, no need for if-else, as returning err does the same thing

[sh0rez]

Could be one line using if err := ...; err != nil