Multiple classes attributes

When having multiple classes, right now it's necessary to write

```
<flow class={"base"+(cond ? " opt" : "")+(cond2 ? " opt2" : "")}/>
```

Here's an alternate syntax proposal for same result (only for class attribute)

```
<flow class={"base" : true, "opt" : cond, "opt2" : cond2 }/>
```

Could also be:

```
<flow class="base" classes={ "opt" : cond, "opt2" : cond2 }/>
```
 
