Add `--jump` flag for intermediate role access

[cnuss]

In the following case:

• $(saml-to assume some-role --headless)

Assume that some-role is an intermediate role that can access other roles.

Add a --jump {some-other-role-arn} flag which will further assume the desired role, e.g.:

$(saml-to assume some-role --headless --jump arn:aws:iam::000000000000:role/some-other-role)

Which would:

• assume some-role using GitHub token
• assume arn:aws:iam::000000000000:role/some-other-role using some-role token
• save/output arn:aws:iam::000000000000:role/some-other-role token

Make --jump able to be specified any number of times for additional additional intermediate roles, e.g.:

$(saml-to assume some-role --headless --jump arn:aws:iam::000000000000:role/some-other-role --jump arn:aws:iam::1111111111111:role/another-role)

Which would:

• assume some-role using GitHub token
• assume arn:aws:iam::000000000000:role/some-other-role using some-role token
• assume arn:aws:iam::1111111111111:role/another-role using arn:aws:iam::000000000000:role/some-other-role token
• save/output arn:aws:iam::1111111111111:role/another-role token