Allow --dir and -d to specify destination directory

[astuyve]

What has been implemented?

Closes #714

Steps to verify

• Run sls help, and note you see both --dir and -d as options
• run sls init fullstack-app -d my-custom-dir and see that the package is fetched from registry and set up in my-custom-dir
• run sls init fullstack-app --dir my-custom-dir and see that the package is fetched from registry and set up in my-custom-dir
• run sls init website -d my-custom-dir and see that the component is initializedin my-custom-dir
• repeat with an initToken from the dashboard, and note that it works as above.

[eahefnawy]

Thanks @astuyve ... could you also add this new option to the docs here:
https://github.com/serverless/components#serverless-init-package-name