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 `
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
* [x] prompt
* [ ] config-angular

## Your Environment

| Executable | Version |
| ---: | :--- |
| `commitlint --version` | 6.2.0 |
| `git --version` | 2.17.0 |
| `node --version` | 8.9.0 |

Start by tracing the prompt package's prompt-cli configuration loading, using issue #100 as context for the existing behavior. Verify how a custom config path is passed to prompt-cli and confirm that configured type and scope enums are loaded instead of the default-only behavior.

Written by the indexing model from the issue text.