This feature has been asked several times, and I can't see any way to fix
the GETENV parsing mess that we have. In this patch the GETENV support is
retired and replaced with a templating system, using the Mustache syntax.
To get back the GETENV feature, our implementation of the Mustache template
system adds support for fetching the template variable values from the OS
environment.
Fixes#555, Fixes#609.
See #500, #477, #278.
