Kirbytext image: new additional params for link,rel,class to avoid duplicates with image params

Old: 
```(image:my.src class:foo bar title:mytitle link:href://foo.baz)```
results in:
```html
<a href="href://foo.baz" title="mytitle" class="foo bar">
<img src="my.src" title="mytitle" class="foo bar">
</a>
```
New: ```(image:my.src class:foo bar title:mytitle link:href://foo.baz linktitle:linky linkclass:boo far linkrel:myrel)```
results in:
```html
<a href="href://foo.baz" title="linky" class="boo far" rel="myrel">
<img src="my.src" title="mytitle" class="foo bar">
</a>
```