Add on_page_body_index event

This event is passed the current collection which is being indexed. Any
content which is returned by the hook is appended to the content for the
search index.

This was necessary for a project where there was some content
programatically added to pages which needed to be included in the search
index.
allow event to modify original text
typo fix and better checking of variable
Merge pull request #859 from triplei/PageIndexEvent

Add on_page_body_index event