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.