Add tag prop to render non-span html elements

[pdxbmw]

This PR creates a prop tag, which adds the ability to define the type of HTML element to render. Alternatively, by making the prop falsy, the component can render text-only.

In my case, I'm using the <Pluralize /> component in a SVG text tag, which doesn't accept span as a child element. With this change, I can either use an acceptable a tag or no element at all.

Note: span is still used by default so this shouldn't affect current users.

[pdxbmw]

My linter added semicolons. Please let me know if you need these removed.