Allow InsertTags for the "emails" validator

This patch allows **InsertTags** for fields with a `eval => array('rgxp'=>'emails')` field validator.

It is not possible to append custom InsertTags to the `recipient` configuration (`tl_form`).

Example recipients:
```html
user1@domain.tld,user2@domain.tld,{{custom::insert::tag}}
```

You can parse the insert tag with the `$GLOBALS['TL_HOOKS']['prepareFormData']` hook.