Indexing plugin as an adapter for end user's own POJO instances

It could be helpful to let end users manipulate information at indexing time with their own code without the need for writing their own indexing plugin. I mentioned this on the dev mailing list (https://www.mail-archive.com/dev@nutch.apache.org/msg31190.html) with some description of my work in progress.

One potential use is to address some of the same concerns that NUTCH-585 discusses regarding an alternative approach to picking and choosing which content to index, but this approach would allow making index time decisions, rather than setting the configuration for all content at the start of the indexing run.