Make `wp help` use a pager

Follow-up to #548.

When using `man`, the output was paged. You could achieve the same effect, without this patch, by running this:

```
wp help core config --color | less -r
```

### less bug

When I run the above command, the output skips the "NAME" header.

Omitting the `--color` flag has no effect.
Changing from iTerm2 to Terminal has no effect.
Piping explicitly instead of from PHP has no effect.

Removing `config` from the command does have an effect. Maybe it's because the synopsis for `core config` is longer than $COLUMNS.