Remember credentials

[thomkrillis]

The generator should remember and re-use provided credentials

ACs:

• generator should prompt user as to whether it should store provided credentials locally
• if stored credentials are found, generator will prompt user for credentials with defaults set to the found credentials

[thomkrillis]

Still unclear whether we should store GH creds or GH and Travis tokens generated from those credentials. We will want to store the AWS credentials somehow.

We will store credentials, not tokens, to simplify user prompts. Just prompt user every time and use found credentials as default values.