Firmware Changelogs?

there ought to be a way to do it from the IDE or from the command line.

There will be - I'm just waiting on Nordic as they have said they are in the process of developing a JavaScript firmware updater (no point duplicating work that'll be done by the manufacturer of the chip :)).

I would like to know what new or deprecated features are changes and what bugs there were that have been squashed.

That zip goes straight to the firmware updater program, so it's not really meant to have any extra info in it.

Proper changelog is here: http://www.espruino.com/ChangeLog

Or if you're interested in what's been done but not yet released, you can see it here: https://github.com/espruino/Espruino/blob/master/ChangeLog