There’s a golang feed parser that might be a starting point. https://github.com/mmcdole/gofeed
Or if you speak javascript, the guy who developed RSS in the first place has developed “River”, which parses/aggregates your feeds for consumption in the browser. http://river4.smallpict.com/2014/09/20/whatIsRiverbrowser.html
Edit: or in python, and this time it’s actually design to output md.
Edit2: in perl: https://github.com/wesleyhales/blog-to-markdown