Cache Markdown'ed HTML in database

This is done via coercion rules. To cache the field 'content' in the
database, include this in your Eve schema:

    {'content': {'type': 'string', 'coerce': 'markdown'},
     '_content_html': {'type': 'string'}}

The `_content_html` field will be filled automatically when saving the
document via Eve.

To display the cached HTML, and fall back to display-time rendering if it
is not there, use `{{ document | markdowned('content') }}` in your template.

Still needs unit testing, a CLI command for regenerating the caches, and
a CLI command for migrating the node type definitions in existing projects.

src/templates/nodes/custom/group/view_embed.pug

@@ -15,7 +15,7 @@
 
 	| {% if node.description %}
 	.node-details-description
-		| {{ node.description | markdown }}
+		| {{ node | markdowned('description') }}
 	| {% endif %}
 
 	section.node-children.group