[core] rules/severity levels

[jack1902]

to: @ryandeivert @chunyong-lin cc: @airbnb/streamalert-maintainers related to: #1171 resolves: #1157

Background

Originally opened under #1171 but the diff got a bit nuts. So i've re-created the original bits but i don't have a test environment for this anymore so can't easily test. #1171 was contentious but i'd rather open this to be merged or closed so others don't ask on Slack if this feature will be enabled or not.

Changes

• [core] Added severity levels to rules (#1157)
• [testing] Updated unit_tests accordingly
• [docs] Updated documentation for the rule so there is a description around severity levels
• [git] Updated .gitignore to ignore .vscode (i'll end up commiting it by mistake)

Testing

Ran ./tests/scripts/unit_test.sh locally and observed errors that are hard to see due to the environment not actually being deployed (AWS Creds are not available nor should be during testing)

[jack1902]

Abandoning this work, as i no longer use StreamAlert and i don't believe this will be merged into master