Support global variables in nunjucks format

[damithc]

v1.8.4

Current: markbind variables are declared in the variables.md as the body of a div element and can be accessed globally. This has some downsides:

• Certain html fragments cannot be used as a variable value. e.g., <div class="indented"> cannot be used as a variable value because it will break the nested div structure of the page due to missing </div> tag.
• Only flat string values can be a variable value. It may be useful to have more structured data values e.g., arrays of strings

Suggested: provide a way to declare variables in an alternate syntax that allows the above. We can use nunjucks variables syntax. However, markbind should raise such variables into the global scope, similar to normal markbind variables.