Add support for begin/end messages with grouped output

Fixes #647

This allows CI systems that support grouping (such as with [GitHub Actions's `::group::` command](https://docs.github.com/en/actions/learn-github-actions/workflow-commands-for-github-actions#grouping-log-lines) and [Azure Devops](https://docs.microsoft.com/en-us/azure/devops/pipelines/scripts/logging-commands?view=azure-devops&tabs=bash#formatting-commands)) to collapse all of the logs for a single task, to improve readability of logs

## Example

The following Taskfile

```
# Taskfile.yml
version: 3
output:
  group:
    begin: "::group::{{ .TASK }}"
    end: "::endgroup::"
tasks:
  default:
    cmds:
      - "echo 'Hello, World!'"
```

Results in the following output
```bash
$ task
task: [default] echo 'Hello, World!'
::group::default
Hello, World!
::endgroup::
```

See [this GitHub Actions job](https://github.com/janslow/task/runs/4811059609?check_suite_focus=true) for a full example

<img width="771" alt="image" src="https://user-images.githubusercontent.com/1253367/149429832-6cb0c1b5-0758-442e-9375-c4daa65771bc.png">
<img width="394" alt="image" src="https://user-images.githubusercontent.com/1253367/149429851-1d5d2ab5-9095-4795-9b57-f91750720d40.png">

task_test.go

@@ -1171,3 +1171,28 @@ func TestIgnoreNilElements(t *testing.T) {
 		})
 	}
 }
+
+func TestOutputGroup(t *testing.T) {
+	const dir = "testdata/output_group"
+	var buff bytes.Buffer
+	e := task.Executor{
+		Dir:    dir,
+		Stdout: &buff,
+		Stderr: &buff,
+	}
+	assert.NoError(t, e.Setup())
+
+	expectedOutputOrder := strings.TrimSpace(`
+task: [hello] echo 'Hello!'
+::group::hello
+Hello!
+::endgroup::
+task: [bye] echo 'Bye!'
+::group::bye
+Bye!
+::endgroup::
+`)
+	assert.NoError(t, e.Run(context.Background(), taskfile.Call{Task: "bye"}))
+	t.Log(buff.String())
+	assert.Equal(t, strings.TrimSpace(buff.String()), expectedOutputOrder)
+}