Implement mechanism for extending control data fields
Implement mechanism for extending control data fields

In line with the refactor, I believe this will be helpful to keeping the Crawler
class as general purpose as possible.

freedomofpress/fingerprint-securedrop,freedomofpress/fingerprint-securedrop,freedomofpress/FingerprintSecureDrop,freedomofpress/FingerprintSecureDrop,freedomofpress/fingerprint-securedrop