Allow asynchronous downloading of data and templates over HTTP or HTTPS

Being able to plug a remote API's data into templates is a need I have for one of my projects, so I came up with this solution. This is a promise-based structure to fetch the required assets and then write out the rendered HTML files as the downloads complete. It's one possible solution for 5thWall/mustache-render#16 on the issue list.

I realize this is quite the hefty change set, particularly with restructuring for promises, so I won't be offended if you'd rather do something else, but I did want to offer it upstream.