Show usage when called without required args

Instead of this error:

```bash
henry@henry-pc: img2iss (master=): which img2iss 
/usr/bin/img2iss
henry@henry-pc: img2iss (master=): img2iss 

fs.js:427
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory '/home/henry/Code/img2iss/lib/../config/config.json'
    at Object.fs.openSync (fs.js:427:18)
    at Object.fs.writeFileSync (fs.js:966:15)
    at Object.module.exports.initialise (/home/henry/Code/img2iss/lib/config.js:47:20)
    at module.exports (/home/henry/Code/img2iss/lib/img2iss.js:38:16)
    at Object.<anonymous> (/home/henry/Code/img2iss/bin/img2iss:4:1)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)

```