Added line nums to code blocks, two new posts

This commit amends previous posts with code blocks to use `#!lang`
syntax instead of `:::lang` since the former adds line numbers to the
code block. The only cases where I've used the latter are output and
interactive Python console sessions. The only gotcha with the line
number versions is that they have to use `pre` instead of `pre-wrap`, so
any wrapping will need to be done by hand.

This post also contains two new posts.