[Button] - should allow to pass an html content

[daniele-zurico]

Currently, the button allows only passing a label element but no a more complex content like a span or whatever other thing.
We should be allowed to do stuff like this:

<button>
   <span>this</span>
   <span> is </span>
   <span> the </span>
   <span> content </span>
</button>

please remember to:

• apply the change
• write the test to prove it
• update the documentation page in storybook
• update the examples in storybook in each section
• update the live version in storybook

git checkout release/1.0.0
git pull
git checkout -b 'feature/button-content'

[daniele-zurico]

closed by #576