Add async execution ability

[romash1408]

Closes #2

[romash1408]

I see now that there should be ability to throw errors from .json.js for callback er, or Promise catch, becose now unhandled promises looks in console like

Also, try with console.error gives

something strange in the middle of webpack output, whithout filename and string of error.
Maybe add error argument to resolve callback, so in .json.js it will look like this?

var fs = require('fs')

module.exports = function(resolve) {
    fs.readdir(__dirname, (err, files) => {
        if (err) return resolve(err)

        // ...

        resolve(null, data)
    })
}