Add `generate_utf8` and update `generate_string`

Add method `generate_utf8`, which returns a string composed randomly from the
entire set of UTF-8 characters. Update method `generate_string` so that it calls
`generate_utf8` and `generate_cjk` when asked for a "utf8" or "cjk" string,
respectively.

Update docstring for method `generate_string` to reflect these changes.
Add tests for method `generate_utf8`