Documented HTML hooks for JavaScript plugin authors

In #981 I added data-column= attributes to the <th> on the table page. These should become part of Datasette’s documented API so JavaScript plugin authors can use them to derive things about the tables shown on a page (`datasette-cluster-map uses them as-of https://github.com/simonw/datasette-cluster-map/issues/18).

[simonw]

In terms of “places to put your plugin content”, the simplest solution I can think of is something like this:

<div id="plugin-content-pre-table"></div>

Alternative designs:

• A documented JavaScript function that returns the CSS selector where plugins should put their content
• A documented JavaScript function that returns a DOM node where plugins should put their content. This would allow the JavaScript to create the element if it does not already exist (though it wouldn’t be obvious WHERE that element should be created)
• Documented JavaScript functions for things like “append this node/HTML to the place-where-plugins-go”

I think the original option - an empty <div> with a known id attribute - is the right one to go with here. It’s the simplest, it’s very easy for custom template authors to understand and it acknowledges that plugins may have all kinds of extra crazy stuff they want to do - like checking in that div to see if another plugin has written to it already, for example.

[simonw]

This is joined with #1191 now, which I’ve bumped from 0.54.