Add option to hide excerpt contents

[Ryan Carpenter]

The standard confluence excerpt plugin includes a selection box to hide the excerpt contents. This is helpful when the excerpt content summarises the page content or duplicates parts of it. It would be nice to keep this option in PolyExcerpt so I can always use the same plugin for excerpts, regardless of whether I want to hide the excerpt content or not. One problem with the standard plugin is that hiding the excerpt content sometimes results in unwanted paragraph breaks (extraneous white space) when displaying the page or when including the page by using the page include macro. I prefer to put the excerpt at the top of the page, where the extra space really interferes with formatting, so avoid this problem would be ideal.