Add excerpts to other posts, and show_excerpt flag

NOTE there were issues with the smart-quote apostrophe in the URL for
	source/_posts/2010-01-27-einstein%E2%80%99s-theory-of-general-relativity-a-personal-description.md

It seems to work fine as is in prod on S3, but locally it won't properly load the index.html file in the directory. I'd get rid of the weird character, but it's in the current WordPress URL.