Add Tabs component

[nastiatikk]

This PR adds the Tabs component This PR is continuation of https://github.com/mobify/stencil/pull/110

Status: Ready for Review Owner: @cole-sanderson Reviewers: @ry5n @jeffkamo @mlworks @nastiatikk @avelinet @yourpalsonja

Todo

• [ ] Create reference design for tabs. Nominate: @jackygilbertson
• [ ] Test accessibility: ARIA roles and VoiceOver/Chrome testing. See this example. Assigned to: @cole-sanderson; nominate @mlworks to assist
• [ ] UI Dev
• [ ] Designer
• [ ] Migrate to a separate Stencil 2.0 repo

[ry5n]

I really want to engineer this so the Tabs are entirely separate from the tabbed content that they control. I think this is essential for flexibility – being able to place the tabs inside of a wrapper component for example (like the BTR site header) and the content somewhere else.

For an example of what that might look like, take the Paper Tabs component from Polymer. They have a separate component that is very much like Scooch that is controlled by the Tabs component.