Fix #159.  Allow config files to be output from external commands.
Fix #159.  Allow config files to be output from external commands.

* Convention: a filename of $(...) is actually a process specification.
* The process is executed in the current directory of the containing config file.
* The standard output of the process is used as the config file content and parsed.
* The process specification is handed to the platform shell for execution.

adobe-apiplatform/user-sync.py,adorton-adobe/user-sync.py,adobe-apiplatform/user-sync.py,adorton-adobe/user-sync.py