From http://blog.pkh.me/p/21-high-quality-gif-with-ffmpeg.html#usage
brew install ffmpeg # need at least 2.6
#!/bin/sh
palette="/tmp/palette.png"
filters="fps=15,scale=320:-1:flags=lanczos"
ffmpeg -v warning -i $1 -vf "$filters,palettegen" -y $palette
ffmpeg -v warning -i $1 -i $palette -lavfi "$filters [x]; [x][1:v] paletteuse" -y $2
./gifenc.sh video.mkv anim.gif