Support async rendering

[Dreamsorcerer]

template() and render_template() should be async functions, or for backwards compatibility new async versions of the functions should be added.

They should call jinja2.Template.render_async(), which also requires enable_async in the environment setup.

See:
https://jinja.palletsprojects.com/en/2.11.x/api/#jinja2.Environment
https://jinja.palletsprojects.com/en/2.11.x/api/#jinja2.Template.render_async
https://jinja.palletsprojects.com/en/2.11.x/api/#async-support

[Dreamsorcerer]

Currently, it looks like the only way to use the async functionality is like this:

template = aiohttp_jinja2.get_env(request.app).get_template("index.jinja")
return web.Response(text=await template.render_async(), content_type="text/html")

[Dreamsorcerer]

Given that passing enable_async breaks the existing functions, maybe a good way to handle backwards compatibility is just to switch the functions when enable_async is passed by the user.

e.g.

if kwargs.get("enable_async", False):
    aiohttp_jinja2.template = aiohttp_jinja2._template_async
    aiohttp_jinja2.render_template = aiohttp_jinja2._render_template_async

Or, something like that. Then all the user has to do is add enable_async=True to the setup() call.

[Dreamsorcerer]

@asvetlov I'm happy to work on this PR. Any thoughts on the correct way to implement it? Does my last comment seem reasonable?

[asvetlov]

I think we need separate functions for async mode.

If you are willing to work on it -- please go ahead