Add additional param to pass arguments to exif

[yetithefoot]

Sometimes need to pass params to command line, for example for excuding some tags from output JSON or format dates.

This pull request is about that. Extra argument was added to function:

var arguments = '--PhotoshopThumbnail --ThumbnailImage';

exif(params.localPath, {}, arguments, function(err, obj) {
    // ...
})

[Yvem]

Hello !

Thanks for this contribution, but could you add tests ?