Add template hook for plugin devs

[erikreagan]

Consider adding in a template hook for 3rd party devs to inject their own fieldtype sample code into the template.

http://buildwithcraft.com/classreference/services/TemplatesService#hook-detail

{% hook 'cheatSheetTemplate' %}
{% if fieldInfo is defined %}
    {% for pluginField in fieldInfo %}
        {{ csMacro.codeBlock(pluginField.title, pluginField.code, pluginField.helpText) }}
    {% endfor %}
{% else %}
    {# final no results html block #}
{% endif %}

Then plugin devs could use the init() method to modify the template context like so:

craft()->templates->hook('cheatSheetTemplate', function(&$context)
{
    $context['fieldInfo'] = array();
}