GCI: Login in CLI using API Authentication Token String

[radiantly]

What is this PR about?

GCI Task: Login in CLI using API Authentication Token String

Features:

• Adds a token argument that can be used to save the EvalAI token.
• Validates the token and checks whether the token is of the right length.
• Tokens can be added or updated using the $ evalai token <your_token> command

Screenshots:

Reviewers:

Please review @RishabhJain2018 @vkartik97