Add repository topics to improve GitHub "searchability"

[iby]

Hey @kylef, thanks for the awesome project. I went searching for it on GitHub and ended up resurrecting one of the old projects in order to find what was that CLI framework I used…

This repo clearly must be on the first page, but it's not:

• https://github.com/search?q=swift+command+line+interface
• https://github.com/search?q=swift+cli

Adding topics/tags would really help more people find this project! ❤️