Add Pre-Processing Option to PMLC

[tajmone]

Please add a preprocessor option to pmlc so that it's possible to create a standalone source document from a project that uses [insert and [insert_code directives and multiple files.

Possibly, there should also be a way to control whether [const parameters should be expanded and replaced or whether they should be preserved in the pre-processed document generated via this option.

This option would come handy in various context, from someone wanting to release a document which is generated as part of a more complex toolchain, to the PML projects themselves, which could generate standalone sample docs and documentation files (for inclusion in releases, projects, etc.) without having to give up the power of [insert and [insert_code directives in their maintenance.

See also Ascidoctor's coalescer script (soon to become an official extension).

[pml-lang]

create a standalone source document

Good idea! A useful feature, indeed.