Add proprietary, non-public advisory datasources to vulntotal

[pombredanne]

It would be great to add support for proprietary data sources in vulntotal. Candidate could include: BlackDuck, vuldb, Snyk, Mend, and several other feeds. The ability to compare them all is a useful feature. The difficulty is that we do not have access to these private data, or to their API specs.

Already tracked:

• Collect vuldb #1199
• Consider safetydb for vulntotal #1108
• Add mend.io DataSource #835

[armijnhemel]

#652