Add support for examples

[seancdavis]

Considerations:

• Should we support themes?
• Is --starter the option for examples, too? Or is there a better name.
• If using separate options, we should validate against using multiple, conflicting options.
• Where should we place the manifest such that adding a new example doesn't require a new version of this script? Or are we okay with versions being tightly tied to the starting options?