feat(cli): Better CLI args validation

Start adding better CLI args validation (#603). Validate log-level to start. This doesn't close the issue.
How about changing

```js
exports.LOG_DISABLE = 'OFF';
exports.LOG_ERROR   = 'ERROR';
exports.LOG_WARN    = 'WARN';
exports.LOG_INFO    = 'INFO';
exports.LOG_DEBUG   = 'DEBUG';
```

in *constants.js* to:

```js
exports.LOG.DISABLE = 'OFF';
exports.LOG.ERROR   = 'ERROR';
exports.LOG.WARN    = 'WARN';
exports.LOG.INFO    = 'INFO';
exports.LOG.DEBUG   = 'DEBUG';
```

Then we can get here `Object.keys(require('constants').LOG).join(', ')` than our knowledge will be in one place `constants.js`, if we add new log level then it immediately appear here.