Fixing -c config option in CLI

This fixes an issue with the -c (--config) option where the specified config file's path would be resolved from the cli.js directory path instead of process execution directory path, causing an error.

For example, if the user is in `~/project-dir/` and runs `eslint -c special-config.json lint-me.js`, the CLI would try to require `special-config.json` from `~/project-dir/node_modules/eslint/lib/special-config.json` instead of the intended `~/project-dir/special-config.json`.

Changing `__dirname` to `process.pwd()` resolves the issue.