Added Viaduct Archfile

Viaduct is an awesome new PaaS hosting service by @adamcooke.
This pull requests adds an `Archfile` which Viaduct parses when creating a new application. This sets a number of commands in order to get Ghost up and running in no time.
I've tested it on both master and stable and it works great.

The build commands, at the bottom of the file, are different when cloning from git than the ZIP file so maybe something could be added to the script that builds the ZIP file on release to add a slightly different file?