fixes and features - mfa and credentials file support, etc

[keen99]

this was work done for myself - to support the needs of my environment. figured I'd share it back in case it's helpful.

• ugly rework of [[ ... =~ ...]] tests that were failing unexpectedly (bash version issue? I gave up trying to diagnose) using a hopefully more portable simple grep
• add support for profiles that are in the credentials file (required for MFA roles)
• remove SESSION_PROFILE code, we need to use the extracted profile for MFA profiles to work
• make session names less generic, easier to id what created the session
• add AWS_EXPIRATION export to allow knowledge of when the token will expire

[cytopia]

@keen99 thank you very much for the effort and contribution. Before merging I will have to thoroughly test this and ensure the code changes will not introduce any issues on us. So please expect some time until merge.

add AWS_EXPIRATION export to allow knowledge of when the token will expire

I've also initially thought about adding this. Be aware if you assume into a login role and then assume further into a permission role, the current AWS limit is 1 hour (unlike only one level of assuming, where you can have 12 hours).

Bare with me, I will report any issues or workings on this one and will also address the shellcheck issues.

[virgilwashere]

I have an updated shellcheck installation method for your travis.yml. will submit when i get in front of computer