Sort keys when dumping to json and yaml

This makes diff'ing stored files easier, e.g.

#!/bin/zsh

# gron: https://github.com/tomnomnom/gron
GRON=/usr/bin/gron
DIFF=/usr/bin/diff
LOCAL="$1"
REMOTE="$2"

"${DIFF}" --color='always' -u --label "a/$REMOTE" --label "b/$REMOTE" <( "$GRON" < "$LOCAL" ) <( "$GRON" < "$REMOTE" ) | less -R