Add a filter which can be used by network admins to disable persistent mode.

To prevent site admins from using persistent mode, add the following code to a custom network-activated plugin:

```php
add_filter( 'disable_comments_allow_persistent_mode', '__return_false' );
```
Updated i18n and extended readme.
Merge branch 'master' of https://github.com/solarissmoke/disable-comments

Conflicts:
	README.md
Allow network administrators to disable persistent mode.
Merge branch 'master' of https://github.com/solarissmoke/disable-comments