[Regex] Support recursive interpolations.

[aercolino]

I call recursive interpolations those where some variables of a template may themselves be templates with more variables and so on, and the interpolation provides a selection of all the variables at any level (but all variables with same name are the same variable) and the substitution is carried on until all provided variables have been interpolated.

This feature will make it a no brainer to interpolate variables starting from high level templates and providing a flat list of variable definitions.

I wonder if it makes sense to preserve in the Regex all previously interpolated variables, so that when new variables are defined using templates with old variables, interpolations could take place immediately and automatically.

[aercolino]

Beware of infinite recursion caused by self referencing variables, directly or hopping through others.