Twig dynamic cache plugin

[rhukster]

One feature I think would be really great to have incorporated into Grav is the ability to cache specific bits of content for defined amounts of time.

https://github.com/asm89/twig-cache-extension

Provides a great twig extension that does just what the doctor ordered. Would be great to create a custom Grav plugin to add this functionality if required.

[rhukster]

This is working and tested, does just what the doctor ordered!