Support password masking in getpass.getpass()

Currently, getpass.getpass() hides all user input when entering a password. This can throw off non-Unix users who are used to passwords being masked with asterisks *. This has led some users to write their own libraries for this functionality.

Proposal:

- Add an optional argument to `getpass.getpass()` for a character to mask user input

Usage Example:

> import getpass
> password = getpass.getpass(mask='*')
Password: **********
> password = getpass.getpass()
Password: