Add print stylesheet

[jasonnrosa]

Add a default print stylesheet for the boilerplate

[JacobLett]

I was working on a theme and saw this comment so added a print stylesheet. If it helps, you can take a look at the one I used to see if something like this could work for the boilerplate: https://gist.github.com/JacobLett/561b2df4c705e9672f04fd9fee929e5c