Didn't work any of this out, I just stuck things together that I found on the web
First, install the things:
brew install imagemagick
brew install ffmpeg
Then, I have a small (600px wide) iTerm window with a big font. Load Quicktime, and record screen (just the area of the terminal)
Trim and save the mov somewhere (lets say ~/Documents/term.mov)
Then, do:
cd ~/Documents
mkdir movanim
cd movanim
# This will render the mov to individual pngs (at 10fps) at a
# width of 500px (which is approximately the size twitter shows them as)
ffmpeg -i ../term.mov -vf scale=500:-1 -r 10 ./ffout%4d.png
# Then this will convert the pngs to a gif (8ms delay between frames loop forever)
# and optimise it as best as it can (the -layers flag)
convert -delay 8 -loop 0 -layers Optimize ffout*.png animation.gif