Add suffixText option

[tommy-mitchell]

Closes #212.

Adds support for a suffixText option/field on a given spinner, and on the stopAndPersist method. Includes docs, types, and tests.

[tommy-mitchell]

I went ahead and updated the example with prefixText and suffixText, among other capabilities. It could be good to record it as another demo gif.

[sindresorhus]

Thanks :)