Add DATAOPTS

This is a bit hacky, but probably is still the simplest and most
elegant solution to the problem of ⛩📰 书社 needing to use different
variables for different stages of a two‐stage build. Note that the core
variables used to manage the two‐stage process (`INCLUDEDIR`,
`BUILDDIR`, `FINDRULES`, `FINDINCLUDERULES`, and `MODE`) can _not_ be
overridden thru this (or any) mechanism.

This commit also adds a new callable variable, `varquote`, for escaping
dollar signs when passing them thru to submakes.