Allowing cloaking when image and text

See https://github.com/joomla/joomla-cms/pull/3955#issuecomment-50634158
Test with
```
<p><a href="mailto:info@mysite.com"><img style="margin: 0px 3px 0px 0px; float:left; width:21px; height:21px;" title="Email to me" src="images/powered_by.png" alt="An email" width="21" height="21" /> Contact us</a><p>
```
and
```
<p><a href="mailto:info@mysite.com?subject=A Text"><img style="margin: 0px 3px 0px 0px; float:left; width:21px; height:21px;" title="Email to me" src="images/powered_by.png" alt="An email" width="21" height="21" /> Contact us</a><p>
```

Will have to be added also to:
https://github.com/joomla/joomla-cms/pull/3967