cli: add "--config" option support

I want move commitlint.config.js to config directory, but I cannot found option like commitlint --config myapp/config/commitlint.config.js.

Expected Behavior

$ commitlint --config myapp/config/commitlint.config.js

Current Behavior

$ commitlint --config myapp/config/commitlint.config.js
/home/xxx/myapp/node_modules/@commitlint/cli/lib/cli.js:50
		throw new Error(`unknown flags: ${arg}`);
		^

Error: unknown flags: --config
    at Object.unknown [as unknownFn] (/home/xxx/myapp/node_modules/@commitlint/cli/lib/cli.js:50:9)
    at setArg (/home/xxx/myapp/node_modules/meow/node_modules/minimist/index.js:56:23)
    at module.exports (/home/xxx/myapp/node_modules/meow/node_modules/minimist/index.js:120:17)
    at module.exports (/home/xxx/myapp/node_modules/meow/index.js:44:13)
    at Object.<anonymous> (/home/xxx/myapp/node_modules/@commitlint/cli/lib/cli.js:54:11)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:458:32)
    at tryModuleLoad (module.js:417:12)
    at Function.Module._load (module.js:409:3)

Affected packages

• cli
• core
• prompt
• config-angular

[lc-soft]

@marionebl OK, I will take some time to review the information you provided and try to solve this issue, but it may take a few days.

[marionebl]

Released via 6.1.0, thanks @lc-soft!