Add sg (Singapore) flavor

- Phone number
- Postal code
- National IC/Foreign ID

P.S. Sorry I am a newb. This is my first pull request. Thank you for your consideration.


For here, I was more thinking of just removing the max_length and min_length from this __init__, and just always pass 6 to the superconstructor call on line 36. Or do you see a reason not to?