add new setting to show image in feed

This setting is optional, and off by default. Since the image is not actually
part of the entry content, this would add it to the beginning of the post if the
post thumbnail is set.
add rss output functions

Checks Settings > Reading. If full text is selected, large image is added to entry content.
If Send Images to RSS plugin is active, email size image is added to entry content.
If Summary is selected, thumbnail is added to description (excerpt).
add rss function
validate feed_image
add help text
modify feed output if featured image exists in post