Added a filter for full text indexation of elements.

Hi,

This one line patch adds a filter to modify full text indexation. This is needed for example to remove some fields, specially internal and management ones, from full text search (see https://groups.google.com/forum/#!topic/omeka-dev/cMPhwyvh75M). This can be used with the patch of the plugin Hide Elements (https://github.com/zerocrates/HideElements/pull/8). 

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management