Add T.TODO(test bool, description string)

The spec [1] suggests "not ok 13 # TODO ...", but if you attempt this
with:

  t.Fail("# TODO foo")

you get an extra hyphen ("not ok 13 - # TODO foo") which is parsed as
a failed test (and not a TODO test).

[1]: http://testanything.org/tap-version-13-specification.html#todo-tests

Author: wking

Makefile

@@ -1,4 +1,4 @@
-TESTS = auto check diagnostic failing known skip writer
+TESTS = auto check diagnostic failing known skip todo writer
 GOPATH = $(CURDIR)/gopath
 
 .PHONY: $(TESTS)

tap.go

@@ -117,6 +117,18 @@ func escapeNewlines(s string) string {
 	return strings.Replace(strings.TrimRight(s, "\n"), "\n", "\n# ", -1)
 }
 
+// TODO indicates a known bug (which is expected to fail).
+func (t *T) TODO(test bool, description string) {
+	// did the test pass or not?
+	ok := "ok"
+	if !test {
+		ok = "not ok"
+	}
+
+	t.printf("%s %d # TODO %s\n", ok, t.nextTestNumber, description)
+	t.nextTestNumber++
+}
+
 // Skip indicates that a test has been skipped.
 func (t *T) Skip(description string) {
 	t.printf("ok %d # SKIP %s\n", t.nextTestNumber, description)

test/todo/main.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"bytes"
+	"io"
+	"os"
+
+	tap "github.com/mndrix/tap-go"
+)
+
+func main() {
+	// collect output for comparison later
+	buf := new(bytes.Buffer)
+	t := tap.New()
+	t.Writer = io.MultiWriter(os.Stdout, buf)
+
+	t.Header(2)
+	t.TODO(false, "bend space and time")
+
+	got := buf.String()
+	t.Ok(got == expected, "skip gave expected output")
+}
+
+const expected = `TAP version 13
+1..2
+not ok 1 # TODO bend space and time
+`