Allow sections to extend other sections

A common pattern is to use a variable in a section to define how values are knit together, and reference other variables in the section to set the values. To allow the structure to be re-used with a different set of values, it should be possible to declare that one section extends another, and define new values to be used within the structure. This requires that the structure be able to refer to keys from "this" section, where "this" section is the section doing the extending.