asciidoc: add missing dependencies

The current asciidoc expression is impure; it relies on several tools to
be found in PATH at runtime. This commit adds a enableStandardFeatures
parameter that pulls in all dependencies and patches asciidoc to contain
full paths to the tools.

enableStandardFeatures defaults to true because asciidoc may attempt to
call them in its default configuration. With all standard features, the
closure size increases from 255 MiB to about 1.3 GiB. Set
enableStandardFeatures = false if you want a minimal asciidoc.