Use `node-stringprep-icu`

Use `node-stringprep-icu` as an _optional dependency_ so that `node-stringprep` (https://github.com/node-xmpp/node-stringprep-icu)  will work on systems where it wouldn't have built properly in the past.

Fixes #45

One this is in I can push as a bug fix version and so developers `node-xmpp-*` code will start working again on non-linux systems.