Feature: import from ~/.aws/credentials

[JoshMcCullough]

For new users, it'd be great to be able to quickly import all existing profiles with their respective key/secret from ~/.aws/credentials, e.g.:

aws-vault import [profile]

If profile is provided, only import that one profile, otherwise import all within the credentials file. If a profile already has been added to aws-vault, warn the user but proceed adding the remaining profiles.