feat: support secure images

[angeloashmore]

• Adds s param support. If the user already has a signature, this allows passing it through.
• Adds a second argument to buildImgixUrl to take in a secret URL token. If this is provided and s is not provided, an s parameter is generated.

If you think these make sense to add to this library, I can update the README.

Related documentation

• Securing Images - Imgix docs on securing image URLs
• Securing URLs Blueprint - Imgix Blueprint docs on how to create a signature