Add auth login subcommand

[mattt]

$ replicate auth login --help
Log in to Replicate

You can find your Replicate API token at https://replicate.com/account

Usage:
  replicate auth login --token-stdin [flags]

Examples:

	# Log in with environment variable
	$ echo $REPLICATE_API_TOKEN | replicate auth login --token-stdin

	# Log in with token file
	$ replicate auth login --token-stdin < path/to/token

Flags:
  -h, --help          help for login
      --token-stdin   Take the token from stdin.