Validate input

Since this is a formatter, we can't validate on input, which I forgot in the issue description.
Therefor, proposed in the MR is:

• Some regex to strip characters the wa.me URL can't handle and massage the value towards the expected format.
• Added composer.json to suggest installing telephone_validation → for actual input validation

These two combined still allow for a zero between the country code and actual number, but that does not seem to be a problem for Whatsapp.

Pushed and will be added to a new 1.0.1 release. Thanks!