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

- [ ] Build tests
- [ ] Write a "How to test this PR" section
I don’t see where `$space` and `$med-h-space` are defined; these aren’t new in Vellum, are they?

Also, the pattern I’d like to follow for configurable variables is to prefix them all with the component name, as with the [Button component](https://github.com/mobify/stencil/blob/master/dist/components/button/_button.scss). These variables can inherit from Vellum defaults, if they exist. I realise this pattern isn’t well-defined for Stencil yet, but it’s one of the things I want to firm up.

In this case, you might define `$alert-padding` and have it inherit from `$h-space` and/or `$v-space`.