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/comment/_macros.pug

@@ -11,7 +11,7 @@
 		.comment-body
 			p.comment-author {{ comment._user.full_name }}
 
-			span {{comment.properties.content_html | safe }}
+			span {{comment.properties._content_html | safe }}
 
 		// TODO: Markdown preview when editing