hugolib: Enhance `.Param` to permit arbitrarily nested parameter references

hugolib: Enhance `.Param` to permit arbitrarily nested parameter references

The Param method currently assumes that its argument is a single,
distinct, top-level key to look up in the Params map. This enhances the
Param method; it will now also attempt to see if the key can be
interpreted as a nested chain of keys to look up in Params.

Fixes #2598