Add vat number rule for es-MX

[arandilopez]

This change adds the correct way to generate Mexican vat numbers (or tax ids, aka RFC in Mexico). In Mexico there are two different type of vat numbers: For business and for individuals.

• For business, a RFC has the format: 3 chars (including Ñ) according to business name initials, following of a date in format YYMMDD, then a unique alphanumeric key made of 3 chars.
• For individuals, a RFC has the format: 4 chars (including Ñ) according to name initials, followed of the birth date in format YYMMDD, then a unique alphanumeric key made of 3 chars

I'm adding this change in order to improve our fixtures set at boxfactura as we depend in the correctness of a tax id.

[vbrazo]

Thanks 👍

[vbrazo]

👍