Fix get terminalWidth in non interactive mode

[vitalymak]

Run in non-interactive mode:

yargs.wrap(yargs.terminalWidth())

Raises:

YError: Invalid first argument. Expected number or null but received undefined.
    at argumentTypeError (/Users/vitaly/Work/n/node_modules/yargs/lib/argsert.js:70:9)
    at /Users/vitaly/Work/n/node_modules/yargs/lib/argsert.js:41:39
    at Array.forEach (native)
    at module.exports (/Users/vitaly/Work/n/node_modules/yargs/lib/argsert.js:35:21)
    at Object.Yargs.self.wrap (/Users/vitaly/Work/n/node_modules/yargs/yargs.js:692:5)
    at Object.<anonymous> (/Users/vitaly/Work/n/tools/cli.js:275:6)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)

Because process.stdout.columns is undefined in non-interactive shell mode.

[bcoe]

thanks for the contribution! we just need to get tests passing on OSX.

[bcoe]

this is breaking tests because some environments report the width as 0 (OSX on Travis specifically).

What if we replace this with:

return typeof process.stdout.columns !== 'undefined' ? process.stdout.columns : null

[vitalymak]

Done