prompt-cli does not work with custom config file paths

Following #100, there is no way to run prompt-cli with a specific config file path. It just assumes the default which is commitlint.config.js under the repository's root.

Expected Behavior

Either:

1. npm run commit --config </path/to/config.js>
2. COMMITLINT_CONFIG=/path/to/config.js npm run commit
3. { config: { '@commitlint/prompt-cli': { config: '/path/to/config.js' } } } in package.json

Current Behavior

prompt-cli runs with no customizations in place (no type/scope enums loaded etc.)

Affected packages

• cli
• core
• prompt
• config-angular

Your Environment

Executable	Version
commitlint --version	6.2.0
git --version	2.17.0
node --version	8.9.0