New Component: Breadcrumb

This is the Breadcrumb component.

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n 

## Changes

- Added _breadcrumb.scss
- Added breadcrumb.json
- Added an alert readme.md
- Added _breadcrumb.dust

## To-Dos

- [x] Build tests
- [x] Write a "How to test this PR" section
- [x] Refine this stencil in accordance with the proposed changes in #59 
- [x] Fix filename typo

## How to test this PR

1. run ```serve``` in the stencil directory
2. navigate to http://localhost:4000/tests/visual/components/breadcrumb/
This variable name is weird. Should it be `active-style`?