Support option interpolation across groups

The option value interpolation feature only works within a group. This is a proposal to extend this feature to also support interpolation across groups.

The current template syntax allows two forms to specify placeholders: $option and ${option}. The bracketed syntax can easily be extended to also support a group specification as ${group.option}. To remain backwards compatible the non-bracketed syntax will not be modified.

This enhancement will be implemented with a simple change to the regular expression used by string.Template that enables the new group aware syntax to be recognized.