Add TEMPLATES_AUTO_RELOAD config key

Currently jinja environment checks for changes in the template file every time you need to render it. But disk I/O is costly and it may be viable to disable this feature in production.

I added `TEMPLATES_AUTO_RELOAD` config option which allows to set `auto_reload` attribute of `jinja2.Environment` to `False` when the application is running in non-debug mode. This allows to make the template rendering a little more efficient because it disables a checking of changing templates file each time before template is rendered.

In some situations, this could give a significant increase of performance (from 60 to 200 rps in my case). Unfortunately, if you want to do the same by overriding of `Flask.jinja_options` attribute, it would be uncomfortable because a reloading of templates is needed for debugging.

This change should not affect existing applications. Also, it might make sense to disable reloading by default.