Implement blackbox in Golang (#250)

* Initial release

docs/gnupg-tips.md

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+GnuPG tips
+==========
+
+# Common error messages
+
+* Message: `gpg: filename: skipped: No public key`
+* Solution: Usually this means there is an item in
+  `.blackbox/blackbox-admins.txt` that is not the name of the key.
+  Either something invalid was inserted (like a filename instead of a
+  username) or a user has left the organization and their key was
+  removed from the keychain, but their name wasn't removed from the
+  blackbox-admins.txt file.
+
+* Message: `gpg: decryption failed: No secret key`
+* Solution: Usually means you forgot to re-encrypt the file with the new key.
+
+* Message: `Error: can't re-encrypt because a key has expired.`
+* Solution: A user's key has expired and can't be used to encrypt any more. Follow the [Replace expired keys](expired-keys.md) page.
+
+FYI: Your repo may use `keyrings/live` instead of `.blackbox`. See "Where is the configuration stored?"
+
+# GnuPG problems
+
+Blackbox is just a front-end to GPG. If you get into a problem with a
+key or file, you'll usually have better luck asking for advice on
+the gnupg users mailing list TODO: Get link to this list
+
+
+The author of Blackbox is not a GnuPG expert. He wrote Blackbox
+because it was better than trying to remember GPG's horrible flag
+names.