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/Makefile

@@ -0,0 +1,33 @@
+BUMP_VERSION := $(GOPATH)/bin/bump_version
+STATICCHECK := $(GOPATH)/bin/staticcheck
+WRITE_MAILMAP := $(GOPATH)/bin/write_mailmap
+
+$(STATICCHECK):
+	go get honnef.co/go/tools/cmd/staticcheck
+
+lint: $(STATICCHECK)
+	go vet ./...
+	$(STATICCHECK)
+
+test: lint
+	@# the timeout helps guard against infinite recursion
+	go test -timeout=250ms ./...
+
+race-test: lint
+	go test -timeout=500ms -race ./...
+
+$(BUMP_VERSION):
+	go get -u github.com/kevinburke/bump_version
+
+$(WRITE_MAILMAP):
+	go get -u github.com/kevinburke/write_mailmap
+
+release: test | $(BUMP_VERSION)
+	$(BUMP_VERSION) --tag-prefix=v minor config.go
+
+force: ;
+
+AUTHORS.txt: force | $(WRITE_MAILMAP)
+	$(WRITE_MAILMAP) > AUTHORS.txt
+
+authors: AUTHORS.txt