Allow anonymous and root users to be excluded from export

While I still think UID0 and UID1 are legitimate to special-case, I wonder if there might be a more generic solution. For example, I could currently use --entity-types to specify all of the entity types except for users. However, that'd be error-prone. If I could negate that option (eg. --exclude-entity-types) then I could export the users I care about separately, then easily ignore user entities altogether the rest of the time.

FYI, I'm using a little work-around for now:

echo Exporting fixture content.
drush content-sync:export --yes --skiplist
echo Deleting exported 'root' user.
rm `grep "value: root@example.com" fixtures/ -rl`
echo Deleting exported 'anonymous' user.
rm `grep "is_anonymous: true" fixtures/ -rl`