Add optional class prop to all components for appending custom classes

[Inventsable]

Similar to style mixins, it would be nice to have custom class props like so:

<Button class="custom-class">
    Click me
</Button>

I've found myself manually setting the style attribute of certain components at times, which tells me that the components aren't flexible enough as is. This might be a much easier shorthand solution to making any stylistic changes to components