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/CHANGELOG.md

@@ -0,0 +1,19 @@
+# Changes
+
+## Version 1.2
+
+Previously, if a Host declaration or a value had trailing whitespace, that
+whitespace would have been included as part of the value. This led to unexpected
+consequences. For example:
+
+```
+Host example       # A comment
+    HostName example.com      # Another comment
+```
+
+Prior to version 1.2, the value for Host would have been "example " and the
+value for HostName would have been "example.com      ". Both of these are
+unintuitive.
+
+Instead, we strip the trailing whitespace in the configuration, which leads to
+more intuitive behavior.