Allow css-class to add multiple class names via multiple mj-class

[dermyhughes]

Currently if you have css-class on amj-class and use multiple mj-class on a component, only the last css-class is actually added to the HTML.

I think it would be good if it would add all the classes.

Example:
Currently-

<mj-class name="foo" css-class="foo"..."
 <mj-class name="bar" css-class="bar"..."


<mj-text mj-class="foo bar"...

will only add the bar class to the HTML.

<tr class="bar">

Ideally it should add both classes:

<tr class="foo bar">

Thanks

[iRyusa]

@kmcb777
I think this would nice to have for the release, and I don't think it would be hard to do with current mj-attributes implem ?

Can you take a look if we can have this for next beta too ?