Release 9.31.0

What's new?

As of 2019-12-24 v9 is entering maintenance mode. Please direct all PR's to master.

But before this happens, 🎄 Merry Christmas: 🎄

🎁 🎁 🎁 🎁 🎁 🎁 🎁 🎁

• Added GetTag functionality to the FieldLevel interface. Thanks @LRichi for the PR
• Extended Isunique functionality to handle the uniqueness of struct fields in an array/slice. Thanks @shihanng for the PR
• Improved documentation for custom functions. Thanks @skateinmars for the PR
• Added e.164 phone number validation. Thanks @taybart for the PR