Add new `waldo auth` command

[eBardX]

waldo auth <user-token>

This command should first validate that the user token is correct by requesting the user’s info at endpoint GET /1.0/users/me with the header Authorization: Token <user-token> specified. If confirmed, the command should print a success line with the user’s name to stdout. Otherwise, it should print an appropriate error message to stderr and exit with failure.

On success(?), the command should also create a YAML file at ~/.waldo/profile.yml containing the user token:

user_token: u-0123456789abcdef