Add checkbox type for prompt

[xuqingfeng]

prompt:
  trigger_next_step:
    type: checkbox
    default: true

...

  - name: Build image
    type: bash
    cmd: |
      ...
      {% if trigger_next_step %}
        echo "Trigger deployment"
        curl -X POST -d '{...}' -H 'Content-Type: application/json' \
          https://pipelines.com/api/webhook/<uuid>
      {% endif %}

[JuhaS]

Done, default for a checkbox is unchecked (false):

Example:

prompt:
 testing:
   type: checkbox
   default: true

[JuhaS]

Closing