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/kevinburke/ssh_config/AUTHORS.txt

@@ -0,0 +1,9 @@
+Carlos A Becker <caarlos0@gmail.com>
+Dustin Spicuzza <dustin@virtualroadside.com>
+Eugene Terentev <eugene@terentev.net>
+Kevin Burke <kevin@burke.dev>
+Mark Nevill <nev@improbable.io>
+Scott Lessans <slessans@gmail.com>
+Sergey Lukjanov <me@slukjanov.name>
+Wayne Ashley Berry <wayneashleyberry@gmail.com>
+santosh653 <70637961+santosh653@users.noreply.github.com>