Document configuration items in manpage

As discussed on [this bug](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=752458) This adds current items to the "CONFIGURATION" section in r2e.1, manually extracted from `rss2email/config.py`.

It can probably become automated if the comments in `CONFIG['DEFAULT']` become a third string component, but I am not sure if it is worth the trouble.