Multi-select dropdown component

[neonwatty]

Here I propose the addition of a simple mult-selector dropdown component as illustrated in the gif below

This PR includes functionality for this component, including the following new files

• multi_select_test.js: a simple controller in src to power multi-select functionality
• multi_select.html: a fixture for testing the component
• multi_select_test.js: a set of tests for the multi-select component fixture
• multi-select.md: a doc outlining usage of the component (as well as a gif illustration multi-selector.gif)

The PR also contains necessary adjustments to several files (e.g., index.js in src and index.html at root) for illustration of the component on the main demo page.

The associated component tests are run along with the others via npm run test.