Require pkginfo only if needed

[chabou]

I'm working on embedding hpm in hyper.
hpm obviously uses your wonderful lib.

And I have a problem with it. Because I'm using webpack to pack hpm in a single standalone file. And in this case pkginfo is not compatible. Requiring it directly throw an exception:

/Users/chabou/Documents/Projets/hyper/bin/cli.js:11586
    throw new Error('Could not find package.json up from ' +
    ^

Error: Could not find package.json up from 125
    at Function.pkginfo.find (/Users/chabou/Documents/Projets/hyper/bin/cli.js:11586:11)
    at Function.pkginfo.read (/Users/chabou/Documents/Projets/hyper/bin/cli.js:11613:22)
    at module.exports (/Users/chabou/Documents/Projets/hyper/bin/cli.js:11559:21)
    at Object.<anonymous> (/Users/chabou/Documents/Projets/hyper/bin/cli.js:11620:1)
    at Object.webpackEmptyContext.keys (/Users/chabou/Documents/Projets/hyper/bin/cli.js:11626:30)
    at __webpack_require__ (/Users/chabou/Documents/Projets/hyper/bin/cli.js:20:30)
    at Object.<anonymous> (/Users/chabou/Documents/Projets/hyper/bin/cli.js:11128:17)
    at __webpack_require__ (/Users/chabou/Documents/Projets/hyper/bin/cli.js:20:30)
    at Object.<anonymous> (/Users/chabou/Documents/Projets/hyper/bin/cli.js:10280:15)
    at Object.<anonymous> (/Users/chabou/Documents/Projets/hyper/bin/cli.js:10336:30)

I found some explanations here: meteor/meteor#6313 (comment)

Offending lines in pkginfo are: https://github.com/indexzero/node-pkginfo/blob/f4e90ac77034ee9eb3dd7aafc56657d7376795a8/lib/pkginfo.js#L129-L133

This previous PR: #97 add some robustness against pkg but this is not enough with webpack.

This PR make it work if getting version is disabled.

I'm not sure if a webpack option could fix it but this PR do not change args features