Add --format=count to `list` subcommands

It's too much hassle currently. See https://github.com/wp-cli/wp-cli/pull/299#issuecomment-20008186

Old:

```
wp post list --post_type=any --format=csv | wc -l | tr -d ' '
```

New:

```
wp post list --post_type=any --format=count
```