Cross account config

[endemics]

Allow automatic role selection for cross-account role by following the source_profile "trail" in .aws/config

This would address https://github.com/RedVentures/oktad/pull/20#issuecomment-339164077

Before adding documentation for this feature, please let me know if you think this use of source_profile is legit or if I'm abusing it (I couldn't find any documentation anywhere explaining how it's meant to be used).