Add support for Spinners (Bootstrap 4.2)

[TheSharpieOne]

https://getbootstrap.com/docs/4.2/components/spinners/

Probably something like:

<Spinner type="grow" size="sm" color="primary">Loading...</Spinner>

would become:

<div class="spinner-grow spinner-grow-sm text-primary" role="status">
  <span class="sr-only">Loading...</span>
</div>

basically (but with how reactstrap builds the className and all):

<div role="status" {...restProps} class="{className} spinner-{type} spinner-{type}-{size} text-{color}">
  {children && <span class="sr-only">{children}</span>}
</div>

[mjdavidson]

Can I do this?