GitLab

* Proof of concept: Adds a new render_custom_dashboard_chart hook and a new 'custom' rendering engine type.
  The new hook can be used by other datasette plugins to render custom charts.
* Also added a "select" type dashboard filter that populates it's list of options from a query
supplied in the plugin config metadata.

Example `render_custom_dashboard_chart` hook:

```python
@hookimpl
async def render_custom_dashboard_chart(chart_display):
    return "<h3>test <b>1</b> 2 3</h3>"
```

Example query filter config:

```yaml
plugins:
  datasette-dashboards:
    project-metrics:
      title: Data³ - workflow metrics
      description: Metrics about projects, tasks and workflows
      layout:
      - [ custom-chart-display, custom-chart-display]
      - [project-events, column-metrics]
      - [project-tasks-state, project-tasks-state]
      - [task-states,task-states ]
    filters:
        project:
          name: Project
          type: select
          query: select phid as key, name as label from Project
```

Default Python JSON encoder does not support dates