Added support for excerpts in feed.

Added some code to set the feed summary based on the excerpt content.

I also a researched a bit and found that some people may want to use the excerpt as feed content instead of the regular post content for various reasons, one being that it forces the rss users to visit their blogpage to read the entire post.
So I added some code to support toggle this on or off and to prevent redundancy there's also a check so the summary isn't set if the excerpt is already used as feed content.

If you think it clutters the codebase I can leave out the "excerpt as content" part and only set the summary so the excerpt gets exposed in some way at least.