Script to move content from one topic to another

This is part of https://www.pivotaltracker.com/story/show/76471962

All artefacts tagged with the source topic are retagged replacing the
source topic with the destination topic.  If search updates for the
artefact are controlled by panopticon, a search update is triggered too.
Would it be worth using `ENV.fetch` here so that it raises an exception if either are blank? http://ruby-doc.org/core-1.9.3/ENV.html#method-c-fetch