Add severity-filter feature

[ansidorov]

Resolve #9

Running scan without .mobsf file:

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

Add .mobsf file with content:

---
- severity-filter:
  - WARNING

And got only WARNING result:

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