Support DUB package manager.

With this, mustache-d can be used with DUB: <https://github.com/rejectedsoftware/dub>

If mustache-d is then registered at <http://registry.vibed.org/publish> (which I can do if you'd like), then other DUB users can use mustache-d in their projects simply by including it in the "dependencies" section of their project's DUB configuration file.