Overview Macro: Add default column "excerpt"

Besides the default metadata ("last modifier", "creator") it should be possible to show the excerpt of a page which is provided by the Excerpt Macro.
This functionality is also provided by other confluence macros to show a brief description of whats going on in this page.

acceptance criteria

• the overview macro has an option to show the excerpt in the result table
• the excerpt is rendered content
• the excerpt column is not sortable
• if the excerpt is shown in the overview, it is also included in the CSV export