integrate with system-provided supervisor

If you already have a system package and possibly unrelated tasks managed by supervisord, you probably do not want to generate the whole boilerplate which would result in duplicate sections for [supervisord] and friends, thus rendering  your configuration "ugly", to say the least.

This patch introduces a new option, "standalone", which can be set to False, resulting in the boilerplate config to be omitted. Under Debian, the resulting file can then simple be moved to /etc/supervisor/conf.d/ and immediately activated, requiring no further editing.