RFC: vulnerability reachability

Vulnerability reachability is to check if vulnerable code is reachable or not.
This is important to help triage vulnerabilities.

Some of the things to consider:
- [x] Collecting introducing/fix commits or patches to find vulnerable functions
- [x] https://github.com/nexB/vulnerablecode/issues/207
- [x] Include commits and patches that introduce a vulnerability
- [x] https://github.com/nexB/vulnerablecode/issues/327
- [ ] Finding the path in the affected package code graph to the vulnerable functions
- [ ] Finding the path in the codebase under analysis that may use the affected package vulnerable functions