Allow a plug-in to perform actions when a new URI is added to the queue

[nevali]

For example, a sameas plug-in might push http://sameas.org/rdf?uri=<encoded URI> into the queue if it doesn't already exist and the queued URI isn't within sameas.org to begin with.

[cgueret]

See https://github.com/bbcarchdev/anansi/tree/ANANSI-59

[nevali]

Let's back this out of develop for now so that other things can happen cleanly.