WIP: Plugin includes

[simonw]

Refs #1191

Next steps:

• Get comfortable that this pattern is the right way to go
• Implement it for all of the other pages, not just the table page
• Add a new set of plugin tests that exercise ALL of these new hook locations
• Document, then ship

[simonw]

Writing the tests will be a bit tricky since we need to confirm that the include_table_top(datasette, database, actor, table) arguments were all passed correctly but the only thing we get back from the plugin is a list of templates. Maybe encode those values into the template names somehow?

[20after4]

Writing the tests will be a bit tricky since we need to confirm that the include_table_top(datasette, database, actor, table) arguments were all passed correctly but the only thing we get back from the plugin is a list of templates. Maybe encode those values into the template names somehow?

Why not return a data structure instead of just a template name?

I've already done some custom hacking to modify datasette but the plugin mechanism you are building here would be much cleaner than what I've built. I'd be happy to help with testing this PR and fleshing it out further if you are still considering merging this.

[simonw]

I think this is missing the _macro.html template file but I have that in my Dropbox.