Add option --config to parse JSON configs

[dbelokon]

Fixes #15.

[menghif]

Hi Diana, thank you for the PR, it looks good :)

2 small things missing:

• Make sure the 'file.json' exists. You can use fs.existsSync(argv.config)
• Make sure that only a json file can be used as input (A folder named 'folder.json' returns an error). You can use fs.statSync(argv.config).isFile()

Thank you!

[dbelokon]

Hey @menghif! I have made the changes you wanted. Let me know if you need anything else 😀