Accept cli args or config value and forward them into `new CLIEngine`

[ryan953]

The eslint CLIEngine class accepts all the same runtime configuration as the cli does if we were to run eslint directly on a set of files.

It would be great if esprint could also accept those same flags on the cli, or even as part of .esprintrc and forward them in to the engine. Then we could use options like --config, --fix or --rules-dir.

Constructor params: https://eslint.org/docs/developer-guide/nodejs-api#cliengine
Cli options: https://eslint.org/docs/user-guide/command-line-interface#options
Looks like we'd want to get the values into this constructor: https://github.com/pinterest/esprint/blob/master/src/LintWorker.js#L3