Add YAML() for generating YAML blocks

Docs in [1].  I've used Go's JSON serializer to avoid external
dependencies, since JSON is a subset of YAML [2].  Prove is currently
choking on the results:

  $ prove test/yaml/test
  test/yaml/test .. Failed 1/2 subtests

  Test Summary Report
  -------------------
  test/yaml/test (Wstat: (none) Tests: 1 Failed: 0)
    Parse errors: Unsupported YAMLish syntax: '{' at /usr/lib64/perl5/5.22.3/TAP/Parser/YAMLish/Reader.pm line 101.

                  Bad plan.  You planned 2 tests but ran 1.
  Files=1, Tests=1,  0 wallclock secs ( 0.01 usr +  0.00 sys =  0.01 CPU)
  Result: FAIL
  $ prove --version
  TAP::Harness v3.35_01 and Perl v5.22.3

but node-tap [3] parses it fine:

  $ tap test/yaml/test
  test/yaml/test ........................................ 2/2
  total ................................................. 2/2

    2 passing (32.15ms)

    ok

  $ tap --version
  11.0.0

[1]: https://testanything.org/tap-version-13-specification.html#yaml-blocks
[2]: http://www.yaml.org/spec/1.2/spec.html
     $ curl -s http://www.yaml.org/spec/1.2/spec.html | grep -B1 JSON | head -n2
             The primary objective of this revision is to bring YAML into compliance
             with JSON as an official subset. YAML 1.2 is compatible with 1.1 for
[3]: http://www.node-tap.org/

Author: wking

util/tap.go

@@ -21,6 +21,7 @@
 package tap // import "github.com/mndrix/tap-go"
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"os"
@@ -151,3 +152,15 @@ func (t *T) Diagnostic(message string) {
 func (t *T) Diagnosticf(format string, a ...interface{}) {
 	t.printf("# "+escapeNewlines(format)+"\n", a...)
 }
+
+// YAML generates a YAML block from the message.
+func (t *T) YAML(message interface{}) error {
+	bytes, err := json.MarshalIndent(message, "  ", "  ")
+	if err != nil {
+		return err
+	}
+	t.printf("  ---\n  ")
+	t.printf(string(bytes))
+	t.printf("\n  ...\n")
+	return nil
+}