add severity-filter feature

[ansidorov]

Resolve #69

Running scan without .njsscan file:

❯ docker run -v $(pwd):/app njsscan /app --json | jq '.nodejs[].metadata.severity'
"INFO"
"INFO"
"WARNING"
"INFO"
"INFO"
"WARNING"
"WARNING"
"WARNING"
"WARNING"
"ERROR"
"ERROR"
"ERROR"
"WARNING"
"ERROR"

Add .njsscan file with content:

---
- severity-filter:
  - ERROR
  - WARNING

And got only WARNING and ERROR results:

❯ docker run -v $(pwd):/app njsscan /app --json | jq '.nodejs[].metadata.severity'
"WARNING"
"WARNING"
"WARNING"
"WARNING"
"WARNING"
"ERROR"
"ERROR"
"ERROR"
"WARNING"
"ERROR"

[ajinabraham]

LGTM 👍