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">

docs/usage.md

@@ -959,6 +959,33 @@ tasks:
  finishes, so you won't have live feedback for commands that take a long time
  to run.
 
+When using the `group` output, you can optionally provide a templated message
+to print at the start of the group. This can be useful for instructing CI
+systems to group all of the output for a given task, such as with [GitHub
+Actions' `::group::` command](https://docs.github.com/en/actions/learn-github-actions/workflow-commands-for-github-actions#grouping-log-lines).
+
+```yaml
+version: '3'
+
+output:
+  group:
+    begin: '::begin::{{.TASK}}'
+    end: '::endgroup::'
+
+tasks:
+  default:
+    cmds:
+      - echo 'Hello, World!'
+    silent: true
+```
+
+```bash
+$ task default
+::begin::default
+Hello, World!
+::endgroup::
+```
+
  The `prefix` output will prefix every line printed by a command with
  `[task-name] ` as the prefix, but you can customize the prefix for a command
  with the `prefix:` attribute: