Render jinja2 templates in async mode

[simonw]

I started playing with this in #404 and got good results but it didn't work in Python 3.5. As of #627 I don't support 3.5 any more so this can go ahead.

Rendering templates in async mode will mean that template plugins can include async code... which opens the door to custom template functions that execute SQL queries!

[simonw]

This is going to be a lot easier than the first attempt because I already modified the .render() method to be async four months ago: fcfcae2#diff-4296b31822a931646fa92cd2d5581521R105

[simonw]

I'm releasing this as Datasette 0.32 - I have a new plugin which takes advantage of it here: https://github.com/simonw/datasette-template-sql