Request: Add support for templates to Markdown card

[ghost]

I'd like the ability to dynamically display the contents of a sensor that contains markdown. This would allow for weather alerts to be posted to the top of my dashboard as they are issued.

      - type: entity-filter
        entities:
          - binary_sensor.display_notifications
        state_filter:
          - 'on'
        card:
          type: markdown
          content: "{{ states('sensor.notifications') }}"
        show_empty: false

In other words, I'd like my markdown card to look like this:

Tornado Warning

The National Weather Service...

==== Instead of

{{ states('sensor.notifications') }}

[c727]

we don't support templates but you can create a custom card

[aaronwolen]

This would be such a neat feature.