Add paramater to the OpenSearch plugin to allow user to specify the amount of page content to return in the results.

Background:

The user previously had Confluence 2.10.3 installed with the 1.0.2-plugins1 version of the OpenSearch plugin installed. When performing searches, a short description appeared in the results.

When this plugin is used with Confluence 3.4.9 with the 1.0.5 or 1.0.2 version of the OpenSearch plugin installed, when performing searches, a long description appears in the results.