Allow to ignore specific Gemfile groups

[fatkodima]

Algorithm:

1. get all groups from Gemfile (gemfile.groups)
2. remove ignored groups from the above (groups_to_check)
3. get all gem specifications for gems in groups to check (parent_dependencies)
4. using breadth-first search, get the remaining gem specifications

Closes #195

[fatkodima]

ping @postmodern @reedloden

[postmodern]

FYI, in the 0.8.0 branch (soon to be merged/released) support for a .bundle-audit.yml configuration file was added with support for a ignore: key to ignore individual advisories by ID.

I'm hesitant about ignoring whole groups, as that could allow vulnerabilities to go un-patched/un-mitigated; attackers don't care if a vulnerable gem is only activated during development if they can still somehow exploit it. Ignoring by individual advisory, where you confirm that you've mitigated it somehow, is much safer.