context_field access plugin

This performs various operations on a field in a context object (right now, specifically a field within the 'data' member of the object). I'm posting this just to get the discussion going on the best way to do this. Right now I'm not satisfied with the way fields are selected from contexts, although it's not like it's totally unviable. May just need a little tweaking. The issue of handling CCK fields (and other array-type fields) correctly still exists, however, and that's something we ought to discuss in this thread.