TUN-3995: Optional --features flag for tunnel run.

These features will be included in the ClientInfo.Features field when
running a named tunnel.

cmd/cloudflared/tunnel/config_test.go

@@ -0,0 +1,13 @@
+package tunnel
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDedup(t *testing.T) {
+	expected := []string{"a", "b"}
+	actual := dedup([]string{"a", "b", "a"})
+	require.Equal(t, expected, actual)
+}