Add configurable notable severity

[linuxdaemon]

This allows you to set the severity of a detection in its config

[pyth0n1c]

Hi folks! I know this PR has been outstanding for a long time, but were are looking to implement it.
In this PR, the severity is set manually. Do you prefer this, or would you prefer it to be set dyanimically based on the risk score? For example, based on the risk score:
0-19: INFORMATIONAL
20-39: LOW
40-59: MEDIUM
60-79: HIGH
CRITICAL:80-100

This means one less field a user will have to set.
Any thoughts?
You can see a relevant link to Enterprise Security Docs here: https://docs.splunk.com/Documentation/ES/7.3.2/User/RiskScoring#:~:text=Risk%20score%20levels%20use%20the%20same%20naming%20convention%20as%20event%20severity.%20You%20can%20assess%20relative%20risk%20scores%20by%20comparing%20hosts%20with%20similar%20roles%20and%20asset%20priority.

[linuxdaemon]

Hey @pyth0n1c, I'm no longer with Etsy so will not be continuing work on these PRs.

[pyth0n1c]

This PR will be closed as the changes are now tracked in the following PR:
#191

Risk Severity will be set automatically based on the risk_score, which is a combination of the impact and confidence fields.