Add social security number detection to secret detection

Description

In response to https://threatpost.com/medical-data-leaked-on-github-due-to-developer-errors/158653/, @theoretick wrote:

OT: I wonder if a social security regex is a worthwhile gitleaks addition \d{3}-\d{2}-\d{4}

Implementation Plan

• Update gitleaks.toml default regexes to check for social security numbers: http://rion.io/2013/09/10/validating-social-security-numbers-through-regular-expressions-2/
• Add testcase for social security number