Found It!  You need to download the

[Visual Studio 2013 SDK][sdk]
=====

Once installed, the project template will appear here:

![demo][demo]

[sdk]: http://www.microsoft.com/en-us/download/details.aspx?id=40758
[demo]: http://i.imgur.com/M6dBU0H.png

Both of these articles talk about how to develop extensions for VS2013, but both presume that you have an extension project type available

* [Extending the Visual Studio Environment][extending]
* [Creating Extensions By Using the VSIX Project Template][vsix]

As of 11/9/13, there are already [544 extensions][gallery] for VS2013 in the gallery, so I'm guessing it's possible, but I can't find the template anywhere.

[extending]: http://msdn.microsoft.com/en-us/library/vstudio/esk3eey8.aspx
[vsix]: http://msdn.microsoft.com/en-us/library/vstudio/dd885241.aspx
[gallery]: http://visualstudiogallery.msdn.microsoft.com/site/search?f%5B0%5D.Type=VisualStudioVersion&f%5B0%5D.Value=12.0&f%5B0%5D.Text=Visual%20Studio%202013