Adds a PhoneNumber type based on google.type.PhoneNumber.

[rofrankel]

This PR currently adds a Markdown schema and protobuf implementatiion for phone number.

If the overall schema looks good, I'll add a JSON Schema implementation as well.

[toumorokoshi]

high-level comment, the content LGTM.

[toumorokoshi]

To be honest I don't see this type being used much - for those who want a phone number and not accept a short code would use a string (which I imagine is many situations, e.g. user accounts).

but for those who need a type with both, this makes sense.