feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/sergi/go-diff/LICENSE

@@ -0,0 +1,20 @@
+Copyright (c) 2012-2016 The go-diff Authors. All rights reserved.
+
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+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
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+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included
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+
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+