Enhancement suggestion: for PageSpeed, automatically create cache directory

Hi there! First of all: _awesome_ script, I found it by mere chance, and it works so flawlessly that it's actually scary...

Now for a suggestion — I've mostly used this script to add `pagespeed` & `brotli` support on `nginx`. `pagespeed` in particular has a gazillion options, but one that pretty much everyone will be using is, obviously, adding the cache directory.

By convention — and your script follows this convention! — `/var/ngx_pagespeed_cache` is the default. However, a common configuration change (at least on Ubuntu/Debian) is to use a `tmpfs`-mounted directory for the cache, say, `/run/ngx_pagespeed_cache` (and just use a symlink to keep the rest of the default configuration working).

This has a problem: when rebooting, as expected, the content of `/run` will be lost, and when `nginx` starts, it expects `/run/ngx_pagespeed_cache` to exist, and because it doesn't exist any more (just a broken symlink pointing to it), `nginx` will fail to run.

On systems using `systemd`, one simple solution is to add a small file under `/usr/lib/tmpfiles.d`, which will tell the system what to create when starting up. In fact, a lot of packages (`mysql`, `php-fpm`, among so many others...) use this method to set up sockets, for example, which are conveniently created on a `tmpfs` volume for added speed.

It would be nice if a future version of `nginx-autoinstall` would be able to create something like that automatically, e.g.:

```
echo "d /run/ngx_pagespeed_cache 0755 www-data www-data - -" > /usr/lib/tmpfiles.d/nginx.conf
ln -s /run/ngx_pagespeed_cache /var/ngx_pagespeed_cache
```

This could be one of the (many) extra options for those interested in having a `tmpfs`-based cache for `pagespeed`...

Keep updating `nginx-autoinstall`, great job so far, and thanks again!