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/ProtonMail/go-crypto/openpgp/internal/encoding/encoding.go

@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package encoding implements openpgp packet field encodings as specified in
+// RFC 4880 and 6637.
+package encoding
+
+import "io"
+
+// Field is an encoded field of an openpgp packet.
+type Field interface {
+	// Bytes returns the decoded data.
+	Bytes() []byte
+
+	// BitLength is the size in bits of the decoded data.
+	BitLength() uint16
+
+	// EncodedBytes returns the encoded data.
+	EncodedBytes() []byte
+
+	// EncodedLength is the size in bytes of the encoded data.
+	EncodedLength() uint16
+
+	// ReadFrom reads the next Field from r.
+	ReadFrom(r io.Reader) (int64, error)
+}