Added config option 'contextSnippetsPerResult'

In my development version of AGWA I used a slightly modified version of TNH which produced 2 context snippets (extracts of text from source documents with search terms highlighted) per search result. The default which is hard-coded in Java source code is 8 which to my eyes is too much.

The following images shows 8 snippets per result:

![search results with 8 context snippets per result](https://f.cloud.github.com/assets/3058613/795406/f2015b9a-ecf4-11e2-9e76-fcb11d7fc9a2.png)

and the next image shows 2 snippets per result:

![search results with 2 context snippets per result](https://f.cloud.github.com/assets/3058613/795408/0d5b9054-ecf5-11e2-8b29-5674ee50b049.png)

The latter is much more in keeping with search results produced by major search engines such as Google, Bing and Yahoo.