Support running with bundled assets

[Joao da Silva]

Window.js applications run like so:

$ windowjs.exe main.js

And then the main.js script can load assets, usually in the same directory and subdirectories.

To make distribution easier, Window.js should support bundling all of the relevant assets into a single .zip file and then executing that directly:

$ windowjs.exe app.zip

APIs like File.readText and so on would need to support looking up files both in the filesystem, and also in the zip bundle if there was one.