Feat/chanhing const DIGITS to 'D'

[Stelchenko]

Overview

I came across the fact that if I need a phone mask for India (+91), the library will allow me to enter a different code (for example, +31), because it believes that 9 is any digit. By changing the DIGITS constant to 'D', we will be able to hard-set the mask, which will contain any numbers.

I also changed the tests and the readme to match the DIGITS = 'D'

[akinncar]

Hey @Stelchenko, thanks for your contribution, and nice find

We need to consider an alternative solution since modifying 9 to D is a significant breaking change.

Also, we may have the same problem if someone wants to indicate that the only accepted digit is D, but the user could type a number instead. (I need to test to make sure)