Added a function to simply validate without formatting numbers

Formatting of comments might be a bit messy, but they're [shown inline here](https://github.com/jordansexton/libphonenumber/commit/e9c3efbe50aaeb6b4266dc4fbe1e293036c5e4cd).

I added and exposed a `validateNumber` function to validate the phone number without formatting it. Internally, it just extracts most of the logic from `formatNumber` which now delegates to `validateNumber`.