rdtool/main_rss.py: add -B to skip posts after a given date

Useful when you're in the middle of the course and you want to
skip post authored, say, less than two days ago, because students
may still be working on such posts.

Yes, in a perfect world students only publish posts when the posts
are ready; however, I routinely publish posts before they are
ready to see how my posts look like, therefore, I won't protest if
students do the same.