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">

internal/output/output.go

@@ -4,6 +4,14 @@ import (
 	"io"
 )
 
-type Output interface {
-	WrapWriter(w io.Writer, prefix string) io.Writer
+
+// Templater executes a template engine.
+// It is provided by the templater.Templater package.
+type Templater interface {
+	// Replace replaces the provided template string with a rendered string.
+	Replace(tmpl string) string
+}
+
+type Output interface {
+	WrapWriter(w io.Writer, prefix string, tmpl Templater) io.Writer
 }