Publish versioned docs

[shawnbot]

One thing that I think we should do as part of #38 is to publish docs to versioned URLs that will never change. For instance, if 1.0.1-beta was the latest version:

• https://primer.github.io/components/v1.0.1-beta/ is created when we publish to npm
• https://primer.github.io/components/ aliases/redirects to the above URL
• https://primer.github.io/components/v2.0.0-rc.0/ is published for the first release candidate (cc: Publish release candidates from CI #152) of 2.0.0

@imgonnarelph was looking at the v1 docs this morning and was confused when colors weren't working for him in 0.0.11-beta. It'd be great to publish previous releases to avoid that sort of issue.

[shawnbot]

Okay, I think I have a better solution for this:

We can set up a deploy stage in our Travis config that only deploys for tags, and would alias the existing deployment from Now's GitHub integration to a versioned subdomain, i.e.:

• Tag v4.0.0-beta → commit 52b7700

• Now aliased that commit when it was merged to master from primer-components-bfbsvbincl.now.sh

• The deploy stage for the tag would find that deployment and alias it to a versioned hostname:

  now alias primer-components-bfbsvbincl.now.sh primer-components-v4-0-0-beta.now.sh

😎