Add support for "introspection variables"

[bvssvni]

When parsing, the meta data parsed so far is available. This could be used to change rules depending on what is already parsed.

• There are strings, bools and numbers and nodes.
• The meta data could be accessed and used as variables

For example, one could parse a number that tells how items follows.

[bvssvni]

This might be out of the scope of this library.