Allow use of # and 0 digit placeholders in fraction masks

[MarkBaker]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Fractional masks were only identified by ? digit placeholders around the /; now # and 0 are also recognised as valid digit placeholders for fractions; although the simplistic approach implemented is simply to replace # and 0 with ?.