New Component: Alert

This is the Alert Box component.

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n 

## Changes

- Added _alert.scss
- Added alert.json
- Added an alert readme.md
- Added alert.dust

## To-Dos

- [x] Build tests
- [ ] Write a "How to test this PR" section
- [x] Refine this stencil in accordance with the proposed changes in #59