Add a cache system

[WarnDa]

Add a way to set in cache some javascript, so for heavy javascript, if a plugin parse it every second, it uses a lot of cpu/ram
So make a cache of 5-10s (configurable) would help this, and save a lot of performances

I'm willing to pay if someone make a good system for this with a cache per javascript, that can be set in javascript_placeholders.yml like this :