Add ability to prepend a string to config files

In cases where multiple applications are being deployed to the same
server, there's a good chance that naming collisions could apply when
copying the files to the configuration directory. If the user specifies
a "supervisord_configure_file_prefix", it will be prepended to every
filename that is generated for a particular application.

For example, I am using "#{application}-#{stage}-" as the prefix in my applications
to ensure no collisions occur.