Create simple subcommand to scaffold folder structure

[henrik-dmg]

To make it easier to get started with SwiftFrame, a subcommand along the lines scaffold would be helpful. It would create subfolders for every supplied device and locale for screenshots, strings and template files.

Its usage could look something like this:

swiftframe scaffold "en-US" "de-DE" "sv" --path "../path/to/target/directory

This is somewhat related to #18