Added option to toggle xdebug for CLI

Composer goes crazy slow when xdebug is enabled on the CLI, so we generally have it disabled on the CLI. I've added a variable that will allow people to disable it on the CLI.
Can you remove this extra newline (to keep consistency with other sections in the README), and also move this variable into the section just after:

```
php_xdebug_default_enable: 1
php_xdebug_coverage_enable: 1
```

(that way all the enable variables are lumped together and more discoverable).