Just found this macro on wowwiki.

/script SelectGossipAvailableQuest(1)
/script CompleteQuest()
/script GetQuestReward()

It saves you the 'click continue' and 'click complete' steps but you still have to right-click on the NPC to bring up the window. This macro will select the first quest you are on and attempt to complete it. I haven't tried it myself and dont know how the last line will work if there are multiple loot options. Don't blame me if you end up with the wrong item :-D.

When im a litte less drowsy I will put together a more robust macro.