Create a system for markdown accessible variables

[dbolack-ab]

I created #3144 for an idea I had and didn't want to vanish before I did something with it but it probably warrants discussion.

This may be a hammer in search of a nail.

This is an idea I had for allowing marked extensions to determine behavior based on brew configuration. It turned into a generic variable system that could be used for anything.

Sample use cases:

Simple macro values can be inserted in the document or used as metadata for a more advanced PDF output system.
e.g. If we build a copyright macro that inserts "Copyright (InitialYear-)CurrentYear", InitialYear could be stored as a brew variable and then passed along in this system.

A marked extension pattern changes (let's say HR) and a user wants to use the previous engine version. The Brew or theme could store "useV3HR" and the extension code could act accordingly without having to roll back the entire engine.

Am I seeing a solution for a problem that might not exist?

[ericscheid]

See also #1400, an ongoing discussion.

[dbolack-ab]

Ironically? That conversation talks all around the part of the problem this takes care of, while this does absolutely nothing in that space. Glue 'em together and you have a whole solution. :)