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.
