misc fixups
add some clarifications to cust.mkd
repo-specific hooks get a bit more oomph
book-promo
rules: add a "tip" about when the ref is not known
mkdoc: fix ability to "compile" specific mkds only
mirroring: allow 'nosync' option for slaves

(people who look at the *code* will realise there's also a 'sync' option!)
remove old syntax for identifiers in favour of markdown-s
misc fixups
mkdoc: use new pdh
new pdh