Implement blackbox in Golang (#250)

* Initial release

docs/file-ops.md

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+How to add/remove a file into the system?
+=========================================
+
+# Adding files:
+
+- If you need to, start the GPG Agent: `eval $(gpg-agent --daemon)`
+- Add the file to the system:
+
+```
+blackbox file add path/to/file.name.key
+
+# If you want to delete the old plaintext:
+blackbox file add --shred path/to/file.name.key
+```
+
+Multiple file names can be specified on the command line:
+
+Example 1: Register 2 files:
+
+```
+blackbox file add --shred file1.txt file2.txt
+```
+
+Example 2: Register all the files in `$DIR`:
+
+```
+find $DIR -type f -not -name '*.gpg' -print0 | xargs -0 blackbox file add
+```
+
+
+# Removing files
+
+This command
+
+```
+blackbox file remove path/to/file.name.key
+```
+
+TODO(tlim): Add examples.
+
+# List files
+
+To see what files are currently enrolled in the system:
+
+```
+blackbox file list
+```
+
+You can also see their status:
+
+```
+blackbox status
+blackbox status just_one_file.txt
+blackbox status --type ENCRYPTED
+```