Allow multiple spines in config

[breezykermo]

In principle, we want to be able to specify multiple spines for the same format in rheo.toml:

[[pdf.spine]]
title = "Book one"
vertebrae = ["1.*.typ"]
merge = true

[[pdf.spine]]
title = "Book two"
vertebrae = ["2.*.typ"]
merge = true

This would produce two PDFs in build/pdf. I don't think there is any reason that the spines need to be exclusive, and I don't think that there should be a limit on how many spines per format we have. But an implementation would need to scope potential issues that arise here.

Ported from #7.