Import referenced documents at build time

Added import references, restoring and improving the functionality of the ‘transclude’ reference that I previously deprecated and removed.

Has two modes, both of which support comments.

• import, transcludes the content of the referenced document as is, without any additional markup or visual cue.
• aside, creates an aside-type block that also includes an outbound link.

Both support an optional comment at the reference-site that is discarded during substitution.

The following snippet (minus the backslash),

  {{ aside::501108be#getting arrested again }\}

Would be substituted with this markup,

  {{ aside::501108be#getting arrested again }}

Which would then be rendered into the following later in the build,

{{ aside::501108be#getting arrested again }}

Both forms of import create a backlink in the referenced document, but only aside creates a visible outward link at the reference-site. This allows documents to be assembled seamlessly where that is a better fit than a call-out, but generally I expect I’ll use the aside mode more often, as I generally prefer for connections to be bi-directional.