Use a Promise wrapper for fs actions and minimist for argv parsing

[kadamwhite]

This PR (which is PR'd against #15, not against master) proposes some final changes to #15 based on my review comments there.

• Wrap callback-based fs actions in a ¶romise to ensure that action completes before we report success
• Add chalk to project dependencies
• Revert the change to auto-generating the script loader namespace from the project directory, in favor of using the --php-namespace command-line argument

To test, run

npx create-react-app --scripts-version git+https://git@github.com/humanmade/react-wp-scripts.git#12-init-proposed-changes --php-namespace DemoNamespace demo-wpscripts-plugin

[rmccue]

@kadamwhite Can you split your php-namespace change out? The rest seems uncontroversial, so we can merge that straight away :)