add explicit hardbreaks option to signature in publisher's letter
set Asciidoctor version so it floats through 1.5.0 releases
print stacktrace for known programming errors
fetch assets more efficiently, support author modifications

- git clone asset repository to avoid HTTP limitations
- allow author repository to be added or removed
- generate any missing repositories
- update spine repository if author repositories change
- fix Ruby block syntax that was breaking ctags (method called on end)
- consolidate logic
switch to using a public URL for the docs submodule