[16.11.24] Added ability to create custom themes

All variables are used in the default template.

The code in the default theme having to do with custom fields is this portions:

     {% for field in custom_fields %}
            <tr>
                <td colspan="99">
                    <div style="font-weight: bold; padding-top: 20px">{{ field.label }}</div>
                    <div>{{ field.text | newline_to_br }}</div>
                </td>
            </tr>
            {% endfor %}

The contents of a custom field are in field.text.

Themes are created using Liquid.