build: Make novelty package opt-in

[jalseth]

This allows Regal to be built without the go-asciisprite dependency.

[srenatus]

I'd like to understand the problem here. Is there a usecase where the go-asciisprite dependency is problematic?

[jalseth]

For internal reasons, it is more work to have more dependencies, so I was hoping to remove this as a hard dependency for the tool. I spoke a bit with @anderseknert offline and and it sounds like the preference is to only have behavior like this as opt-out rather than opt-in, which doesn't help my use case. Going to close this for now.

[anderseknert]

Eh, change of heart. If this makes things a little easier for you @jalseth , we can live with the fact that go build won't give you the festive output format. It's a secret easter egg anyway, and this way it'll be even more secret 😎