Async rendering support

[Dreamsorcerer]

-- This is branched from the typing PR, so merge that first to reduce the diff --

This adds render_string_async() and render_template_async() functions.
It also modifies the @template decorator to use the async rendering when enable_async=True has been passed to the setup.

Fixes #333

[codecov]

Codecov Report

Merging #393 (61295a2) into master (858bd57) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   96.02%   96.36%   +0.33%     
==========================================
  Files           3        3              
  Lines         151      165      +14     
  Branches       18       19       +1     
==========================================
+ Hits          145      159      +14     
  Misses          6        6              

Flag	Coverage Δ
unit	95.15% <100.00%> (+0.44%)	⬆️

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Impacted Files	Coverage Δ
aiohttp_jinja2/__init__.py	95.27% <100.00%> (+0.58%)	⬆️

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[asvetlov]

Please fix the merge conflict

[Dreamsorcerer]

OK, went a bit weird for a moment. All sorted now.

[asvetlov]

Awesome, I'll prepare a new release with this feature.

@Dreamsorcerer would you get write access to the repo (GitHub Maintainer role precisely).

I don't expect much work here but don't want to be a bus factor and shuffle off routine procedures on somebody else :)
Honestly, asyncio + aiohttp consumes almost all my spare time

[Dreamsorcerer]

Yep, go ahead. Happy to keep things going.

[asvetlov]

Great!
An invitation is sent.
I would suggest you maintain https://github.com/aio-libs/aiohttp-mako
The library is pretty similar to aiohttp-jinja2 but uses the mako template engine (which is simpler than jinja2 actually).
It needs some housekeeping work (drop py3.5, add tests for 3.8-3.9, maybe something also -- I don't remember).