Support table allow-listing

[simonw]

Currently all tables are exposed. Allow plugin configuration to say "only expose these tables and views".

This will be particularly useful for explicitly designing a GraphQL API, since that can be done by carefully creating SQL views and then exposing just them.

[simonw]

Config design:

{
  "plugins": {
    "datasette-graphql": {
      "databases": {
        "github": {
          "tables": ["issues", "repos"]
        }
      }
    }
  }
}

This also leaves space for supporting multiple databases later - #9 - and as a starting point can show an error if you try to start up with more than one database configured since only one is currently supported.

[simonw]

I don't need this for the first non-alpha release - it makes more sense to implement this along with canned queries in #16.