Implement blackbox in Golang (#250)

* Initial release

docs/why-is-this-important.md

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+Why encrypt your secrets?
+=========================
+
+OBVIOUSLY we don't want secret things like SSL private keys and
+passwords to be leaked.
+
+NOT SO OBVIOUSLY when we store "secrets" in a VCS repo like Git or
+Mercurial, suddenly we are less able to share our code with other
+people. Communication between subteams of an organization is hurt. You
+can't collaborate as well. Either you find yourself emailing
+individual files around (yuck!), making a special repo with just the
+files needed by your collaborators (yuck!!), or just deciding that
+collaboration isn't worth all that effort (yuck!!!).
+
+The ability to be open and transparent about our code, with the
+exception of a few specific files, is key to the kind of collaboration
+that DevOps and modern IT practitioners need to do.