Implement blackbox in Golang (#250)

* Initial release

docs/advanced.md

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+Advanced Techniques
+===================
+
+
+# Using Blackbox without a repo
+
+If the files are copied out of a repo they can still be decrypted and
+edited. Obviously edits, changes to keys, and such will be lost if
+they are made outside the repo. Also note that commands are most
+likely to only work if run from the base directory (i.e. the parent to
+the .blackbox directory).
+
+Without a repo, all commands must be run from the same directory
+as the ".blackbox" directory.  It might work otherwise but no
+promises.
+
+
+# Mixing gpg 1.x/2.0 and 2.2
+
+WARNING: Each version of GnuPG uses a different, and incompatible,
+binary format to store the keychain.  When Blackbox was originally
+created, I didn't know this.  Things are mostly upwards compatible.
+That said, if you have some admins with GnuPG 1.x and others with GnuPG 2.2,
+you may corrupt the keychain.
+
+A future version will store the keychain in an GnuPG-approved
+version-neutral format.
+
+
+# Having gpg and gpg2 on the same machine
+
+NOTE: This is not implemented at this time. TODO(tlim) Use GPG to find
+the binary.
+
+In some situations, team members or automated roles need to install gpg
+2.x alongside the system gpg version 1.x to catch up with the team's gpg
+version. On Ubuntu 16, you can ```apt-get install gnupg2``` which
+installs the binary gpg2. If you want to use this gpg2 binary, run every
+blackbox command with GPG=gpg2.
+
+For example:
+
+```
+GPG=gpg2 blackbox_postdeploy
+```
+