feat: add an isTTY check

[Fishrock123]

This prevents the code from having a negative impact on piped stdio.

Refs: nodejs/node#6456 (comment)
Refs: nodejs/node#1771 (comment)

@bcoe

[coveralls]

Coverage decreased (-25.0%) to 75.0% when pulling 50944ba on Fishrock123:isTTY into 7d0c45e on yargs:master.

[Fishrock123]

Uhhhh

idk how to actually test this against a TTY so I'm just gona fake it I guess.

[Fishrock123]

@bcoe lmk if it needs a better test. I can't think of one off-hand though that won't break once we patch the issue.

[coveralls]

Coverage remained the same at 100.0% when pulling b7193b3 on Fishrock123:isTTY into 7d0c45e on yargs:master.

[bcoe]

@Fishrock123 is there a way for us to differentiate between spawn/exec's default pipe behavior, and an application being executed with |?

[bcoe]

@Fishrock123 as an alternative option, perhaps document how yargs uses this module in the README, as a "Word of Warning":

hold off logging until the last moment, and set blocking to true only once you know you're about to exit.

[Fishrock123]

is there a way for us to differentiate between spawn/exec's default pipe behavior, and an application being executed with |?

No, it's literally the same as far as I know.

Maybe if we tweak the stdio settings? But I doubt any of that helps because then there's no way of checking it.. :/ https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_options_stdio

as an alternative option, perhaps document how yargs uses this module in the README, as a "Word of Warning":

I mean we should probably have that too..