feat: implement darwin launch agent (#192)

* feat: sketch out periodic command

* feat: sketch out periodic command for macOS

* feat: implement darwin's launch agent

* refactor: better way to run on darwin

Make sure we have code that builds on all platforms.

* fix(run): max 10 URLs with darwin in unattended mode

* feat: add support for seeing/streaming logs

* feat: implement the status command and add usage hints

* feat(periodic): run onboarding if needed

* fix: no too confusing function names

* fix: s/periodic/autorun/

Discussed earlier this morning with @hellais.

* fix: we cannot show logs before Big Sur

Bug reported by @hellais.

internal/autorun/autorun.go

@@ -0,0 +1,49 @@
+// Package autorun contains code to manage automatic runs
+package autorun
+
+import "sync"
+
+const (
+	// StatusScheduled indicates that OONI is scheduled to run
+	// periodically in the background.
+	StatusScheduled = "scheduled"
+
+	// StatusStopped indicates that OONI is not scheduled to
+	// run periodically in the background.
+	StatusStopped = "stopped"
+
+	// StatusRunning indicates that OONI is currently
+	// running in the background.
+	StatusRunning = "running"
+)
+
+// Manager manages automatic runs
+type Manager interface {
+	LogShow() error
+	LogStream() error
+	Start() error
+	Status() (string, error)
+	Stop() error
+}
+
+var (
+	registry map[string]Manager
+	mtx      sync.Mutex
+)
+
+func register(platform string, manager Manager) {
+	defer mtx.Unlock()
+	mtx.Lock()
+	if registry == nil {
+		registry = make(map[string]Manager)
+	}
+	registry[platform] = manager
+}
+
+// Get gets the specified autorun manager. This function
+// returns nil if no autorun manager exists.
+func Get(platform string) Manager {
+	defer mtx.Unlock()
+	mtx.Lock()
+	return registry[platform]
+}