Scan Multiple AWS accounts via AssumeRole

[kmcquade]

What does this PR do?

• Adds multi-account scan support. Fixes Add cross-account download functionality #96.
• Includes a scan_account method that can be used for Lambda deployments (related to Deployment through lambda? #39)
• Bumps to version 0.4.0

What gif best describes this PR or how it makes you feel?

Completion checklist

• Additions and changes have unit tests
• Python Unit tests, Pylint, security testing, and Integration tests are passing.
• Javascript tests are passing (npm test)
• If the UI contents or JavaScript files have been modified, generate a new example report (npm build and python3 ./utils/generate_example_report.py)
• The pull request has been appropriately labeled using the provided PR labels

[kmcquade]

@yegorski - I went through and tested this out. Works great. We can also use the scan_account method to address #39 (Lambda support).

Let me know if it looks good to you. If so I will merge and we can release 0.4.0 :)

[kmcquade]

@yegorski - due to lots of demand, having tested this myself, and having addressed your previous suggestions I am going to go ahead and merge this PR and bump the version to 0.4.0.

I've updated the documentation to include instructions on scanning as well.

If you run into any issues, let me know. And feel free to open a new GitHub issue.

[yegorski]

@kmcquade sounds great, thank you! Can't wait to use it!