Added vulnerability reference link information.

[iamdharmesh]

Description of the Change

PR adds vulnerability reference link information to provide the functionality to users to view more details on vulnerability. Users can now get reference link along with other vulnerability information by adding --reference to the wp-cli vuln command. eg: wp vuln plugin-status --reference

Closes #84

How to test the Change

1. Configure this scanner repo and configure the API provider
2. Run vuln commands by adding the --reference flag and make sure that reference information is there. example: wp vuln status --reference, wp vuln theme-status --reference
3. Try different formats like JSON and CSV and make sure that reference information is there. (add --format=csv)
4. Test with different API providers (Wordfence, WPScan, Patchstack) and verify that it works well with all providers.

Changelog Entry

Added - Vulnerability reference link information.

Credits

Props @iamdharmesh @bmarshall511

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[iamdharmesh]

Note: Wordfence test failure is already fixed in #90 and it is not related to this PR.