Easier way to access ValueSource given docid [LUCENE-4541]

I was working on improving some sample code and I needed to access a value via a ValueSource given a docid in search results. I was disappointed to see how many steps were needed. Incidentally, Solr had to solve the same problem, and it's got ValueSourceAugmenter for this. I propose a small utility class be created to do something similar, such that the client can merely create an instance and then call a method with a doc id to retrieve the value.

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Migrated from [LUCENE-4541](https://issues.apache.org/jira/browse/LUCENE-4541) by David Smiley (@dsmiley)

Start by reviewing Lucene's ValueSource API and Solr's ValueSourceAugmenter, both named in the issue, to understand the existing steps for retrieving a value by document ID. Done means providing a small utility that lets a client create an instance and retrieve the ValueSource value with a doc ID.

Written by the indexing model from the issue text.