metadata.json support for per-database and per-table information

[simonw]

Every database and every table should be able to support the following optional metadata:

title
description
description_html
license
license_url
source
source_url

If description_html is provided it over-rides description and will be displayed unescaped.

[simonw]

Here's an example metadata.json file illustrating custom per-database and per-
table metadata:

{
    "title": "Overall datasette title",
    "description_html": "This is a <em>description with HTML</em>.",
    "databases": {
        "db1": {
            "title": "First database",
            "description": "This is a string description & has no HTML",
            "license_url": "http://example.com/",
    		"license": "The example license",
            "queries": {
            	"canned_query": "select * from table1 limit 3;"
            },
            "tables": {
                "table1": {
                    "title": "Custom title for table1",
                    "description": "Tables can have descriptions too",
                    "source": "This has a custom source",
                    "source_url": "http://example.com/"
                }
            }
        }
    }
}

[simonw]

Needs documentation, see #166