feat(passive): allow marking a flow as passive

A flow that is marked as passive, will not produce the warning
"No destination specified; this entire flow went nowhere."

If a task is set to passive, but a `merge` or `dest` option has
been specified, the build will stop.
feat(passive): allow marking a flow as passive

A flow that is marked as passive, will not produce the warning
"No destination specified; this entire flow went nowhere."

If a task is set to passive, but a `merge` or `dest` option has
been specified, the build will stop.