Improve blog setup

* All posts under blog/
* Index @ blog/index.html
* Only show excerpt on index with "continue reading" link
* Date, name formatting improvements (better for humans)

It could probably still use some style tweaks but I feel better about
it.

Moving forward, we'll use the "excerpt" feature of Jekyll with the
default separator, which is just 2 newlines. So the first paragraph will
be special. Alternatively you can specify excerpt, but we'll want to fix
the layout so that gets added in.