Add jekyll-rss plugin by Assaf Gelber

https://github.com/agelber/jekyll-rss/commit/1a5971ca9137015533ec0766f89b5450a8ebad30
Remove double-slash from RSS links

Fix slash-ification for earlier Ruby versions, or something.
RSS uses subtitle for description, if possible

If not, uses the raw, non-HTML-ified excerpt. Markdown parsing begone.
Set author in config
Switch Markdown parser to kramdown for footnotes
Don't quine li or strong tags

<li> adds a new line when it's used with <p> (which it is).

<strong> is just too long and distracting.
Quine style works better with footnotes

Don't quine in the superscript; don't quine the back-link.
Superscripts get some padding, don't increase line height
Add separator and padding before footnotes
Post: "Timers, Clocks, and Cocoa"