support outputting gdt test scenarios as Markdown

[jaypipes]

We'd like to be able to publish gdt test scenarios as nicely-formatted Markdown that will essentially be the content for a sort of "guided check".

Given the gdt test scenario YAML:

name: var-save-restore
description: a scenario that tests variable save/restore across multiple test specs
tests:
  - exec: echo 42
    var-stdout: VAR_STDOUT

  - exec: echo $$VAR_STDOUT
    var-rc: VAR_RC
    assert:
      out:
        is: 42

  - exec: echo $$VAR_RC
    assert:
      out:
        is: 0

The Markdown output should look like:

# var-save-restore

a scenario that tests variable save/restore across multiple test specs

## Test 1. `echo 42`

```sh
VAR_STDOUT=$(echo 42)
echo $VAR_STDOUT
```

Expect:

```
42
```

## Test 2. `echo $VAR_STDOUT`

```sh
echo $VAR_STDOUT
VAR_RC=$?
```

Expect:

```
42
```

## Test 3. `echo $VAR_RC`

```sh
echo $VAR_RC
```

Expect:

```
0
```