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).
Christopher Allen Lane