Implement blackbox in Golang (#250)

* Initial release

docs/git-tips.md

@@ -0,0 +1,22 @@
+GIT tips
+========
+
+
+# Configure git to show diffs in encrypted files
+
+It's possible to tell Git to decrypt versions of the file before running them through `git diff` or `git log`. To achieve this do:
+
+- Add the following to `.gitattributes` at the top of the git repository:
+
+```
+*.gpg diff=blackbox
+```
+
+- Add the following to `.git/config`:
+
+```
+[diff "blackbox"]
+    textconv = gpg --use-agent -q --batch --decrypt
+````
+
+Commands like `git log -p file.gpg` and `git diff master --` will display as expected.