Callback could be optional

[deybydjimenez]

Hi, I noticed if you don't define a callback on the usage it will throw an exception. Can you make it optional? I think sometimes you don't really need a callback (which is my current scenario)

jsonConcat({
    src: ["appVars.json", "userVars.json"],
    dest: "./config.json"
})

It's annoying to have an empty callback and ESLinter is driving me crazy!

[GochoMugo]

@deybydjimenez Could you open a PR for it?