Add examples of `space-evenly` and `gap`

[jsnkuhn]

Since the creation of this game a few new properties and value have been added to flexbox. space-evenly is now an option for align and justify, and the gap can now be used to mark a specific sized gap between flex items. Both of these could use examples.

[Thomas Park]

Thanks for the suggestion @jsnkuhn. For now I'm adding space-evenly to the popup docs, and I'll look to add levels for these in the future.

[Thomas Park]

Added 1244eee