While setting up this site, I was getting irritated with external links navigating away from the site. There are a couple of ways to resolve this and I started with a dumb brute force version of my own, but quickly realized there had to be a better way. I found a great Gem plugin by Keith Mifsud that solved the whole problem for me.
https://rubygems.org/gems/jekyll-target-blank
You can find the documentation of the 2 minor edits this requires on 
Keith’s website.