Add modal component molecule

[evsheffield]

We've developed this component internally but it could be a nice addition to this library. Some features it should support:

• Allows you to provide header, body, and footer, and composes them all with the relevant AuModal... components and a backdrop
• Include an AuClose in the top right of the modal
• Provides a prop for an onClose handler for clicking on the backdrop or close icon.
• Support for all semantic props (i.e. info, primary, etc.)