Make CLI configuration token secure

## Description
Configuring the CLI requires users to specify the token with the `--token` flag.
This causes the token to stick around in the command line history, which can be easily viewed by someone.
(e.g. just `$ history | grep "exercism configure --token"` can bring my token right in front of the attacker)

## Solution
It would be safer if the equivalent of Python's getpass library is implemented to obtain input (or just plain STDIN). That way, the token is taken as input from a prompt and is not stored in the commands history.

## Additional Notes
Not that it's a huge security risk for users with personal computers, but it would be helpful for shared machines.