Add an option to switch versions of the docs

[mischah]

Hej,

I would love to add a version switcher for the yeoman docs: https://github.com/yeoman/yeoman/issues/1485

Proposal

Add a versionSwitcher property into the template settings like:

"templates": {
    "name": "Doc Template",
    "footerText": "NHN Entertainment Frontend Development Lab",
    "versionSwitcher": true
  }

If templates.versionSwitcher is true and we have the version number from the package.jsonvia opts.package we replace the version number in the left column with a select element to switch versions.

Mockup